Difference between revisions of "DBI (manpage)"
Jump to navigation
Jump to search
m (→Navbar: moved TOC) |
m (Reverted edits by Dash McWayne (Talk); changed back to last version by Woozle) |
| (One intermediate revision by one other user not shown) | |
(No difference)
| |
Latest revision as of 20:09, 21 August 2008
computing: software: documentation: manpages: DBIm
Transcription and reformatting from manpage is incomplete |
Manpage
NAME
DBI - Database independent interface for Perl
SYNOPSIS
use DBI;
@driver_names = DBI->available_drivers;
@data_sources = DBI->data_sources($driver_name, \%attr);
$dbh = DBI->connect($data_source, $username, $auth, \%attr);
$rv = $dbh->do($statement);
$rv = $dbh->do($statement, \%attr);
$rv = $dbh->do($statement, \%attr, @bind_values);
$ary_ref = $dbh->selectall_arrayref($statement);
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement);
$ary_ref = $dbh->selectrow_arrayref($statement);
$hash_ref = $dbh->selectrow_hashref($statement);
$sth = $dbh->prepare($statement);
$sth = $dbh->prepare_cached($statement);
$rc = $sth->bind_param($p_num, $bind_value);
$rc = $sth->bind_param($p_num, $bind_value, $bind_type);
$rc = $sth->bind_param($p_num, $bind_value, \%attr);
$rv = $sth->execute;
$rv = $sth->execute(@bind_values);
$rv = $sth->execute_array(\%attr, ...);
$rc = $sth->bind_col($col_num, \$col_variable);
$rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);
@row_ary = $sth->fetchrow_array;
$ary_ref = $sth->fetchrow_arrayref;
$hash_ref = $sth->fetchrow_hashref;
$ary_ref = $sth->fetchall_arrayref;
$ary_ref = $sth->fetchall_arrayref( $slice, $max_rows );
$hash_ref = $sth->fetchall_hashref( $key_field );
$rv = $sth->rows;
$rc = $dbh->begin_work;
$rc = $dbh->commit;
$rc = $dbh->rollback;
$quoted_string = $dbh->quote($string);
$rc = $h->err;
$str = $h->errstr;
$rv = $h->state;
$rc = $dbh->disconnect;
The synopsis above only lists the major methods and parameters.
GETTING HELP
If you have questions about DBI, or DBD driver modules, you can get help from the dbi-usersspam@spamperlspam.spamorg mailing list. You can get help on subscribing and using the list by emailing dbi-users-helpspam@spamperlspam.spamorg.
(To help you make the best use of the dbi-users mailing list, and any other lists or forums you may use, I strongly recommend that you read "How To Ask Questions The Smart Way" by Eric Raymond: <http://www.catb.org/~esr/faqs/smart-questions.html>)
The DBI home page at <http://dbi.perl.org/> is always worth a visit and includes an FAQ and links to other resources.
Before asking any questions, reread this document, consult the archives and read the DBI FAQ. The archives are listed at the end of this document and on the DBI home page. An FAQ is installed as a DBI::FAQ module so you can read it by executing "perldoc DBI::FAQ". However the DBI::FAQ module is currently (2004) outdated relative to the online FAQ on the DBI home page.
This document often uses terms like references, objects, methods. If you're not familar with those terms then it would be a good idea to read at least the following perl manuals first: perlreftut, perldsc, perllol, and perlboot.
Please note that Tim Bunce does not maintain the mailing lists or the web page (generous volunteers do that). So please don't send mail directly to him; he just doesn't have the time to answer questions personally. The dbi-users mailing list has lots of experienced people who should be able to help you if you need it. If you do email Tim he's very likely to just forward it to the mailing list.
NOTES
This is the DBI specification that corresponds to the DBI version 1.46.
The DBI is evolving at a steady pace, so it's good to check that you have the latest copy.
The significant user-visible changes in each release are documented in the DBI::Changes module so you can read them by executing "perldoc DBI::Changes".
Some DBI changes require changes in the drivers, but the drivers can take some time to catch up. Newer versions of the DBI have added features that may not yet be supported by the drivers you use. Talk to the authors of your drivers if you need a new feature that's not yet supported.
Features added after DBI 1.21 (February 2002) are marked in the text with the version number of the DBI release they first appeared in.
Extensions to the DBI API often use the "DBIx::*" namespace. See "Naming Conventions and Name Space". DBI extension modules can be found at <http://search.cpan.org/search?mode=module&query=DBIx>. And all modules related to the DBI can be found at <http://search.cpan.org/search?query=DBI&mode=all>.
DESCRIPTION
The DBI is a database access module for the Perl programming language. It defines a set of methods, variables, and conventions that provide a consistent database interface, independent of the actual database being used.
It is important to remember that the DBI is just an interface. The DBI is a layer of "glue" between an application and one or more database driver modules. It is the driver modules which do most of the real work. The DBI provides a standard interface and framework for the drivers to operate within.
Architecture of a DBI Application
|<- Scope of DBI ->|
.-. .--------------. .-------------.
.-------. | |---| XYZ Driver |---| XYZ Engine |
| Perl | | | `--------------' `-------------'
| script| |A| |D| .--------------. .-------------.
| using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
| DBI | |I| |I| `--------------' `-------------'
| API | | |...
|methods| | |... Other drivers
`-------' | |...
`-'
The API, or Application Programming Interface, defines the call interface and variables for Perl scripts to use. The API is implemented by the Perl DBI extension.
The DBI "dispatches" the method calls to the appropriate driver for actual execution. The DBI is also responsible for the dynamic loading of drivers, error checking and handling, providing default implementations for methods, and many other non-database specific duties.
Each driver contains implementations of the DBI methods using the private interface functions of the corresponding database engine. Only authors of sophisticated/multi-database applications or generic library functions need be concerned with drivers.
Notation and Conventions
The following conventions are used in this document:
$dbh Database handle object
$sth Statement handle object
$drh Driver handle object (rarely seen or used in applications)
$h Any of the handle types above ($dbh, $sth, or $drh)
$rc General Return Code (boolean: true=ok, false=error)
$rv General Return Value (typically an integer)
@ary List of values returned from the database, typically a row of data
$rows Number of rows processed (if available, else -1)
$fh A filehandle
undef NULL values are represented by undefined values in Perl
\%attr Reference to a hash of attribute values passed to methods
Note that Perl will automatically destroy database and statement handle objects if all references to them are deleted.
Outline Usage
To use DBI, first you need to load the DBI module:
use DBI;
use strict;
(The "use strict;" isn't required but is strongly recommended.)
Then you need to "connect" to your data source and get a handle for that connection:
$dbh = DBI->connect($dsn, $user, $password,
{ RaiseError => 1, AutoCommit => 0 });
Since connecting can be expensive, you generally just connect at the start of your program and disconnect at the end.
Explicitly defining the required "AutoCommit" behaviour is strongly recommended and may become mandatory in a later version. This determines whether changes are automatically committed to the database when executed, or need to be explicitly committed later.
The DBI allows an application to "prepare" statements for later execu tion. A prepared statement is identified by a statement handle held in a Perl variable. We'll call the Perl variable $sth in our examples.
The typical method call sequence for a "SELECT" statement is:
prepare,
execute, fetch, fetch, ...
execute, fetch, fetch, ...
execute, fetch, fetch, ...
for example:
$sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");
$sth->execute( $baz );
while ( @row = $sth->fetchrow_array ) {
print "@row\n";
}
The typical method call sequence for a non-"SELECT" statement is:
prepare,
execute,
execute,
execute.
for example:
$sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");
while(<CSV>) {
chomp;
my ($foo,$bar,$baz) = split /,/;
$sth->execute( $foo, $bar, $baz );
}
The "do()" method can be used for non repeated non-"SELECT" statement (or with drivers that don't support placeholders):
$rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");
To commit your changes to the database (when "AutoCommit" is off):
$dbh->commit; # or call $dbh->rollback; to undo changes
Finally, when you have finished working with the data source, you should "disconnect" from it:
$dbh->disconnect;
General Interface Rules & Caveats
The DBI does not have a concept of a "current session". Every session has a handle object (i.e., a $dbh) returned from the "connect" method. That handle object is used to invoke database related methods.
Most data is returned to the Perl script as strings. (Null values are returned as "undef".) This allows arbitrary precision numeric data to be handled without loss of accuracy. Beware that Perl may not preserve the same accuracy when the string is used as a number.
Dates and times are returned as character strings in the current default format of the corresponding database engine. Time zone effects are database/driver dependent.
Perl supports binary data in Perl strings, and the DBI will pass binary data to and from the driver without change. It is up to the driver implementors to decide how they wish to handle such binary data.
Most databases that understand multiple character sets have a default global charset. Text stored in the database is, or should be, stored in that charset; if not, then that's the fault of either the database or the application that inserted the data. When text is fetched it should be automatically converted to the charset of the client, presumably based on the locale. If a driver needs to set a flag to get that behaviour, then it should do so; it should not require the application to do that.
Multiple SQL statements may not be combined in a single statement handle ($sth), although some databases and drivers do support this (notably Sybase and SQL Server).
Non-sequential record reads are not supported in this version of the DBI. In other words, records can only be fetched in the order that the database returned them, and once fetched they are forgotten.
Positioned updates and deletes are not directly supported by the DBI. See the description of the "CursorName" attribute for an alternative.
Individual driver implementors are free to provide any private functions and/or handle attributes that they feel are useful. Private driver functions can be invoked using the DBI "func()" method. Private driver attributes are accessed just like standard attributes.
Many methods have an optional "\%attr" parameter which can be used to pass information to the driver implementing the method. Except where specifically documented, the "\%attr" parameter can only be used to pass driver specific hints. In general, you can ignore "\%attr" parameters or pass it as "undef".
Naming Conventions and Name Space
The DBI package and all packages below it ("DBI::*") are reserved for use by the DBI. Extensions and related modules use the "DBIx::" namespace (see <http://www.perl.com/CPAN/modules/by-module/DBIx/>). Package names beginning with "DBD::" are reserved for use by DBI database drivers. All environment variables used by the DBI or by individual DBDs begin with ""DBI_"" or ""DBD_"".
The letter case used for attribute names is significant and plays an important part in the portability of DBI scripts. The case of the attribute name is used to signify who defined the meaning of that name and its values.
Case of name Has a meaning defined by
------------ ------------------------
UPPER_CASE Standards, e.g., X/Open, ISO SQL92 etc (portable)
MixedCase DBI API (portable), underscores are not used.
lower_case Driver or database engine specific (non-portable)
It is of the utmost importance that Driver developers only use lower case attribute names when defining private attributes. Private attribute names must be prefixed with the driver name or suitable abbreviation (e.g., ""ora_"" for Oracle, ""ing_"" for Ingres, etc).
SQL - A Query Language
Most DBI drivers require applications to use a dialect of SQL (Structured Query Language) to interact with the database engine. The "Standards Reference Information" section provides links to useful information about SQL.
The DBI itself does not mandate or require any particular language to be used; it is language independent. In ODBC terms, the DBI is in "pass-thru" mode, although individual drivers might not be. The only requirement is that queries and other statements must be expressed as a single string of characters passed as the first argument to the "prepare" or "do" methods.
For an interesting diversion on the real history of RDBMS and SQL, from the people who made it happen, see:
http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html
Follow the "Full Contents" then "Intergalactic dataspeak" links for the SQL history.
Placeholders and Bind Values
Some drivers support placeholders and bind values. Placeholders, also called parameter markers, are used to indicate values in a database statement that will be supplied later, before the prepared statement is executed. For example, an application might use the following to insert a row of data into the SALES table:
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
or the following, to select the description for a product:
SELECT description FROM products WHERE product_code = ?
The "?" characters are the placeholders. The association of actual values with placeholders is known as binding, and the values are referred to as bind values.
Note that the "?" is not enclosed in quotation marks, even when the placeholder represents a string. Some drivers also allow placeholders like ":"name and ":"n (e.g., ":1", ":2", and so on) in addition to "?", but their use is not portable.
With most drivers, placeholders can't be used for any element of a statement that would prevent the database server from validating the statement and creating a query execution plan for it. For example:
"SELECT name, age FROM ?" # wrong (will probably fail)
"SELECT name, ? FROM people" # wrong (but may not 'fail')
Also, placeholders can only represent single scalar values. For example, the following statement won't work as expected for more than one value:
"SELECT name, age FROM people WHERE name IN (?)" # wrong
"SELECT name, age FROM people WHERE name IN (?,?)" # two names
When using placeholders with the SQL "LIKE" qualifier, you must remember that the placeholder substitutes for the whole string. So you should use ""... LIKE ? ..."" and include any wildcard characters in the value that you bind to the placeholder.
Null Values
Undefined values, or "undef", can be used to indicate null values. However, care must be taken in the particular case of trying to use null values to qualify a "SELECT" statement. Consider:
SELECT description FROM products WHERE product_code = ?
Binding an "undef" (NULL) to the placeholder will not select rows which have a NULL "product_code"! Refer to the SQL manual for your database engine or any SQL book for the reasons for this. To explicitly select NULLs you have to say ""WHERE product_code IS NULL"" and to make that general you have to say:
... WHERE (product_code = ? OR (? IS NULL AND product_code IS NULL))
and bind the same value to both placeholders. Sadly, that more general syntax doesn't work for Sybase and MS SQL Server. However on those two servers the original ""product_code = ?"" syntax works for binding nulls.
Performance
Without using placeholders, the insert statement shown previously would have to contain the literal values to be inserted and would have to be re-prepared and re-executed for each row. With placeholders, the insert statement only needs to be prepared once. The bind values for each row can be given to the "execute" method each time it's called. By avoiding the need to re-prepare the statement for each row, the application typically runs many times faster. Here's an example:
my $sth = $dbh->prepare(q{
INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
}) or die $dbh->errstr;
while (<>) {
chomp;
my ($product_code, $qty, $price) = split /,/;
$sth->execute($product_code, $qty, $price) or die $dbh->errstr;
}
$dbh->commit or die $dbh->errstr;
See "execute" and "bind_param" for more details.
The "q{...}" style quoting used in this example avoids clashing with quotes that may be used in the SQL statement. Use the double-quote like "qq{...}" operator if you want to interpolate variables into the string. See "Quote and Quote-like Operators" in perlop for more details.
See also the "bind_columns" method, which is used to associate Perl variables with the output columns of a "SELECT" statement.
THE DBI PACKAGE AND CLASS
In this section, we cover the DBI class methods, utility functions, and the dynamic attributes associated with generic DBI handles.
DBI Constants
Constants representing the values of the SQL standard types can be imported individually by name, or all together by importing the special ":sql_types" tag.
The names and values of all the defined SQL standard types can be produced like this:
foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
printf "%s=%d\n", $_, &{"DBI::$_"};
}
These constants are defined by SQL/CLI, ODBC or both. "SQL_BIGINT" is (currently) omitted, because SQL/CLI and ODBC provide conflicting codes.
See the "type_info", "type_info_all", and "bind_param" methods for possible uses.
Note that just because the DBI defines a named constant for a given data type doesn't mean that drivers will support that data type.
DBI Class Methods
The following methods are provided by the DBI class:
"parse_dsn"
($scheme, $driver, $attr_string, $attr_hash, $driver_dsn) = DBI->parse_dsn($dsn)
or die "Can't parse DBI DSN '$dsn'";
Breaks apart a DBI Data Source Name (DSN) and returns the individual parts. If $dsn doesn't contain a valid DSN then parse_dsn() returns an empty list.
$scheme is the first part of the DSN and is currently always 'dbi'. $driver is the driver name, possibly defaulted to $ENV{DBI_DRIVER}, and may be undefined. $attr_string is the optional attribute string, which may be undefined. If $attr_string is true then $attr_hash is a reference to a hash containing the parsed attribute names and values. $driver_dsn is the last part of the DBI DSN string.
The parse_dsn() method was added in DBI 1.43.
="connect"
$dbh = DBI->connect($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect($data_source, $username, $password, \%attr)
or die $DBI::errstr;
Establishes a database connection, or session, to the requested $data_source. Returns a database handle object if the connection succeeds. Use "$dbh->disconnect" to terminate the connection.
If the connect fails (see below), it returns "undef" and sets both $DBI::err and $DBI::errstr. (It does not explicitly set $!.) You should generally test the return status of "connect" and "print $DBI::errstr" if it has failed.
Multiple simultaneous connections to multiple databases through multiple drivers can be made via the DBI. Simply make one "connect" call for each database and keep a copy of each returned database handle.
The $data_source value must begin with ""dbi:"driver_name":"". The driver_name specifies the driver that will be used to make the connection. (Letter case is significant.)
As a convenience, if the $data_source parameter is undefined or empty, the DBI will substitute the value of the environment variable "DBI_DSN". If just the driver_name part is empty (i.e., the $data_source prefix is ""dbi::""), the environment variable "DBI_DRIVER" is used. If neither variable is set, then "connect" dies.
Examples of $data_source values are:
dbi:DriverName:database_name
dbi:DriverName:database_name@hostname:port
dbi:DriverName:database=database_name;host=hostname;port=port
There is no standard for the text following the driver name. Each driver is free to use whatever syntax it wants. The only requirement the DBI makes is that all the information is supplied in a single string. You must consult the documentation for the drivers you are using for a description of the syntax they require. (Where a driver author needs to define a syntax for the $data_source, it is recommended that they follow the ODBC style, shown in the last example above.)
If the environment variable "DBI_AUTOPROXY" is defined (and the driver in $data_source is not ""Proxy"") then the connect request will automatically be changed to:
$ENV{DBI_AUTOPROXY};dsn=$data_source
"DBI_AUTOPROXY" is typically set as ""dbi:Proxy:hostname=...;port=..."". If $ENV{DBI_AUTOPROXY} doesn't begin with '"dbi:"' then "dbi:Proxy:" will be prepended to it first. See the DBD::Proxy documentation for more details.
If $username or $password are undefined (rather than just empty), then the DBI will substitute the values of the "DBI_USER" and "DBI_PASS" environment variables, respectively. The DBI will warn if the environment variables are not defined. However, the everyday use of these environment variables is not recommended for security reasons. The mechanism is primarily intended to simplify testing. See below for alternative way to specify the username and password.
"DBI->connect" automatically installs the driver if it has not been installed yet. Driver installation either returns a valid driver handle, or it dies with an error message that includes the string ""install_driver"" and the underlying problem. So "DBI->connect" will die on a driver installation failure and will only return "undef" on a connect failure, in which case $DBI::errstr will hold the error message. Use "eval { ... }" if you need to catch the ""install_driver"" error.
The $data_source argument (with the ""dbi:...:"" prefix removed) and the $username and $password arguments are then passed to the driver for processing. The DBI does not define any interpretation for the contents of these fields. The driver is free to interpret the $data_source, $username, and $password fields in any way, and supply whatever defaults are appropriate for the engine being accessed. (Oracle, for example, uses the ORACLE_SID and TWO_TASK environment variables if no $data_source is specified.)
The "AutoCommit" and "PrintError" attributes for each connection default to "on". (See "AutoCommit" and "PrintError" for more information.) However, it is strongly recommended that you explicitly define "AutoCommit" rather than rely on the default. The "PrintWarn" attribute defaults to on if $^W is true, i.e., perl is running with warnings enabled.
The "\%attr" parameter can be used to alter the default settings of "PrintError", "RaiseError", "AutoCommit", and other attributes. For example:
$dbh = DBI->connect($data_source, $user, $pass, {
PrintError => 0,
AutoCommit => 0
});
The username and password can also be specified using the attributes "Username" and "Password", in which case they take precedence over the $username and $password parameters.
You can also define connection attribute values within the $data_source parameter. For example:
dbi:DriverName(PrintWarn=>1,PrintError=>0,Taint=>1):...
Individual attributes values specified in this way take precedence over any conflicting values specified via the "\%attr" parameter to "connect".
The "dbi_connect_method" attribute can be used to specify which driver method should be called to establish the connection. The only useful values are 'connect', 'connect_cached', or some specialized case like 'Apache::DBI::connect' (which is automatically the default when running within Apache).
Where possible, each session ($dbh) is independent from the transactions in other sessions. This is useful when you need to hold cursors open across transactions--for example, if you use one session for your long lifespan cursors (typically read-only) and another for your short update transactions.
For compatibility with old DBI scripts, the driver can be specified by passing its name as the fourth argument to "connect" (instead of "\%attr"):
$dbh = DBI->connect($data_source, $user, $pass, $driver);
In this "old-style" form of "connect", the $data_source should not start with ""dbi:driver_name:"". (If it does, the embedded driver_name will be ignored). Also note that in this older form of "connect", the "$dbh->{AutoCommit}" attribute is undefined, the "$dbh->{PrintError}" attribute is off, and the old "DBI_DBNAME" environment variable is checked if "DBI_DSN" is not defined. Beware that this "old-style" "connect" will soon be withdrawn in a future version of DBI.
"connect_cached"
$dbh = DBI->connect_cached($data_source, $username, $password)
or die $DBI::errstr;
$dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
or die $DBI::errstr;
"connect_cached" is like "connect", except that the database handle returned is also stored in a hash associated with the given parameters. If another call is made to "connect_cached" with the same parameter values, then the corresponding cached $dbh will be returned if it is still valid. The cached database handle is replaced with a new connection if it has been disconnected or if the "ping" method fails.
Note that the behaviour of this method differs in several respects from the behaviour of persistent connections implemented by Apache::DBI.
Caching connections can be useful in some applications, but it can also cause problems, such as too many connections, and so should be used with care.
The cache can be accessed (and cleared) via the "CachedKids" attribute.
"available_drivers"
@ary = DBI->available_drivers;
@ary = DBI->available_drivers($quiet);
Returns a list of all available drivers by searching for "DBD::*" modules through the directories in @INC. By default, a warning is given if some drivers are hidden by others of the same name in earlier directories. Passing a true value for $quiet will inhibit the warning.
"installed_versions"
DBI->installed_versions;
@ary = DBI->installed_versions;
%hash = DBI->installed_versions;
Calls available_drivers() and attempts to load each of them in turn
using install_driver(). For each load that succeeds the driver
name and version number are added to a hash. When running under
DBI::PurePerl drivers which appear not be pure-perl are ignored.
When called in array context the list of successfully loaded
drivers is returned (without the 'DBD::' prefix).
When called in scalar context a reference to the hash is returned
and the hash will also contain other entries for the "DBI" version,
"OS" name, etc.
When called in a void context the installed_versions() method will
print out a formatted list of the hash contents, one per line.
Due to the potentially high memory cost and unknown risks of load-
ing in an unknown number of drivers that just happen to be
installed on the system, this method is nor recommended for general
use. Use available_drivers() instead.
The installed_versions() method is primarily intended as a quick
way to see from the command line what's installed. For example:
perl -MDBI -e 'DBI->installed_versions'
The installed_versions() method was added in DBI 1.38.
"data_sources"
@ary = DBI->data_sources($driver);
@ary = DBI->data_sources($driver, \%attr);
Returns a list of data sources (databases) available via the named
driver. If $driver is empty or "undef", then the value of the
"DBI_DRIVER" environment variable is used.
The driver will be loaded if it hasn't been already. Note that if
the driver loading fails then data_sources() dies with an error
message that includes the string ""install_driver"" and the under-
lying problem.
Data sources are returned in a form suitable for passing to the
"connect" method (that is, they will include the ""dbi:$driver:""
prefix).
Note that many drivers have no way of knowing what data sources
might be available for it. These drivers return an empty or incom-
plete list or may require driver-specific attributes.
There is also a data_sources() method defined for database handles.
"trace"
DBI->trace($trace_setting)
DBI->trace($trace_setting, $trace_filename)
$trace_setting = DBI->trace;
The "DBI->trace" method sets the global default trace settings and
returns the previous trace settings. It can also be used to change
where the trace output is sent.
There's a similar method, "$h->trace", which sets the trace set-
tings for the specific handle it's called on.
See the "TRACING" section for full details about the DBI's powerful
tracing facilities.
DBI Utility Functions
In addition to the DBI methods listed in the previous section, the DBI
package also provides several utility functions.
These can be imported into your code by listing them in the "use"
statement. For example:
use DBI qw(neat data_diff);
Alternatively, all these utility functions (except hash) can be
imported using the ":utils" import tag. For example:
use DBI qw(:utils);
"data_string_desc"
$description = data_string_desc($string);
Returns an informal description of the string. For example:
UTF8 off, ASCII, 42 characters 42 bytes
UTF8 off, non-ASCII, 42 characters 42 bytes
UTF8 on, non-ASCII, 4 characters 6 bytes
UTF8 on but INVALID encoding, non-ASCII, 4 characters 6 bytes
UTF8 off, undef
The initial "UTF8" on/off refers to Perl's internal SvUTF8 flag.
If $string has the SvUTF8 flag set but the sequence of bytes it
contains are not a valid UTF-8 encoding then data_string_desc()
will report "UTF8 on but INVALID encoding".
The "ASCII" vs "non-ASCII" portion shows "ASCII" if all the charac-
ters in the string are ASCII (have code points <= 127).
The data_string_desc() function was added in DBI 1.46.
"data_string_diff"
$diff = data_string_diff($a, $b);
Returns an informal description of the first character difference
between the strings. If both $a and $b contain the same sequence of
characters then data_string_diff() returns an empty string. For
example:
Params a & b Result
------------ ------
'aaa', 'aaa'
'aaa', 'abc' 'Strings differ at index 2: a[2]=a, b[2]=b'
'aaa', undef 'String b is undef, string a has 3 characters'
'aaa', 'aa' 'String b truncated after 2 characters'
Unicode characters are reported in "\x{XXXX}" format. Unicode code
points in the range U+0800 to U+08FF are unassigned and most likely
to occur due to double-encoding. Characters in this range are
reported as "\x{08XX}='C'" where "C" is the corresponding latin-1
character.
The data_string_diff() function only considers logical characters
and not the underlying encoding. See "data_diff" for an alterna-
tive.
The data_string_diff() function was added in DBI 1.46.
"data_diff"
$diff = data_diff($a, $b);
$diff = data_diff($a, $b, $logical);
Returns an informal description of the difference between two
strings. It calls "data_string_desc" and "data_string_diff" and
returns the combined results as a multi-line string.
For example, "data_diff("abc", "ab\x{263a}")" will return:
a: UTF8 off, ASCII, 3 characters 3 bytes
b: UTF8 on, non-ASCII, 3 characters 5 bytes
Strings differ at index 2: a[2]=c, b[2]=\x{263A}
If $a and $b are identical in both the characters they contain and
their physical encoding then data_diff() returns an empty string.
If $logical is true then physical encoding differences are ignored
(but are still reported if there is a difference in the charac-
ters).
The data_diff() function was added in DBI 1.46.
"neat"
$str = neat($value);
$str = neat($value, $maxlen);
Return a string containing a neat (and tidy) representation of the
supplied value.
Strings will be quoted, although internal quotes will not be
escaped. Values known to be numeric will be unquoted. Undefined
(NULL) values will be shown as "undef" (without quotes).
If the string is flagged internally as utf8 then double quotes will
be used, otherwise single quotes are used and unprintable charac-
ters will be replaced by dot (.).
For result strings longer than $maxlen the result string will be
truncated to "$maxlen-4" and ""...'"" will be appended. If $maxlen
is 0 or "undef", it defaults to $DBI::neat_maxlen which, in turn,
defaults to 400.
This function is designed to format values for human consumption.
It is used internally by the DBI for "trace" output. It should typ-
ically not be used for formatting values for database use. (See
also "quote".)
"neat_list"
$str = neat_list(\@listref, $maxlen, $field_sep);
Calls "neat" on each element of the list and returns a string con-
taining the results joined with $field_sep. $field_sep defaults to
", ".
"looks_like_number"
@bool = looks_like_number(@array);
Returns true for each element that looks like a number. Returns
false for each element that does not look like a number. Returns
"undef" for each element that is undefined or empty.
"hash"
$hash_value = DBI::hash($buffer, $type);
Return a 32-bit integer 'hash' value corresponding to the contents
of $buffer. The $type parameter selects which kind of hash algo-
rithm should be used.
For the technically curious, type 0 (which is the default if $type
isn't specified) is based on the Perl 5.1 hash except that the
value is forced to be negative (for obscure historical reasons).
Type 1 is the better "Fowler / Noll / Vo" (FNV) hash. See
<http://www.isthe.com/chongo/tech/comp/fnv/> for more information.
Both types are implemented in C and are very fast.
This function doesn't have much to do with databases, except that
it can be handy to store hash values in a database.
DBI Dynamic Attributes
Dynamic attributes are always associated with the last handle used
(that handle is represented by $h in the descriptions below).
Where an attribute is equivalent to a method call, then refer to the
method call for all related documentation.
Warning: these attributes are provided as a convenience but they do
have limitations. Specifically, they have a short lifespan: because
they are associated with the last handle used, they should only be used
immediately after calling the method that "sets" them. If in any
doubt, use the corresponding method call.
$DBI::err
Equivalent to "$h->err".
$DBI::errstr
Equivalent to "$h->errstr".
$DBI::state
Equivalent to "$h->state".
$DBI::rows
Equivalent to "$h->rows". Please refer to the documentation for the
"rows" method.
$DBI::lasth
Returns the DBI object handle used for the most recent DBI method
call. If the last DBI method call was a DESTROY then $DBI::lasth
will return the handle of the parent of the destroyed handle, if
there is one.
METHODS COMMON TO ALL HANDLES
The following methods can be used by all types of DBI handles.
"err"
$rv = $h->err;
Returns the native database engine error code from the last driver
method called. The code is typically an integer but you should not
assume that.
The DBI resets $h->err to undef before most DBI method calls, so
the value only has a short lifespan. Also, for most drivers, the
statement handles share the same error variable as the parent
database handle, so calling a method on one handle may reset the
error on the related handles.
If you need to test for individual errors and have your program be
portable to different database engines, then you'll need to deter-
mine what the corresponding error codes are for all those engines
and test for all of them.
A driver may return 0 from err() to indicate a warning condition
after a method call. Similarly, a driver may return an empty string
to indicate a 'success with information' condition. In both these
cases the value is false but not undef. The errstr() and state()
methods may be used to retrieve extra information in these cases.
See "set_err" for more information.
"errstr"
$str = $h->errstr;
Returns the native database engine error message from the last DBI
method called. This has the same lifespan issues as the "err"
method described above.
The returned string may contain multiple messages separated by new-
line characters.
The errstr() method should not be used to test for errors, use
err() for that, because drivers may return 'success with informa-
tion' or warning messages via errstr() for methods that have not
'failed'.
See "set_err" for more information.
"state"
$str = $h->state;
Returns a state code in the standard SQLSTATE five character for-
mat. Note that the specific success code 00000 is translated to
any empty string (false). If the driver does not support SQLSTATE
(and most don't), then state will return "S1000" (General Error)
for all errors.
The driver is free to return any value via "state", e.g., warning
codes, even if it has not declared an error by returning a true
value via the "err" method described above.
The state() method should not be used to test for errors, use err()
for that, because drivers may return a 'success with information'
or warning state code via errstr() for methods that have not
'failed'.
"set_err"
$rv = $h->set_err($err, $errstr);
$rv = $h->set_err($err, $errstr, $state);
$rv = $h->set_err($err, $errstr, $state, $method);
$rv = $h->set_err($err, $errstr, $state, $method, $rv);
Set the "err", "errstr", and "state" values for the handle. This
method is typically only used by DBI drivers and DBI subclasses.
If the "HandleSetErr" attribute holds a reference to a subroutine
it is called first. The subroutine can alter the $err, $errstr,
$state, and $method values. See "HandleSetErr" for full details.
If the subroutine returns a true value then the handle "err",
"errstr", and "state" values are not altered and set_err() returns
an empty list (it normally returns $rv which defaults to undef, see
below).
Setting "err" to a true value indicates an error and will trigger
the normal DBI error handling mechanisms, such as "RaiseError" and
"HandleError", if they are enabled, when execution returns from the
DBI back to the application.
Setting "err" to "" indicates an 'information' state, and setting
it to "0" indicates a 'warning' state. Setting "err" to "undef"
also sets "errstr" to undef, and "state" to "", irrespective of the
values of the $errstr and $state parameters.
The $method parameter provides an alternate method name for the
"RaiseError"/"PrintError"/"PrintWarn" error string instead of the
fairly unhelpful '"set_err"'.
The "set_err" method normally returns undef. The $rv parameter
provides an alternate return value.
Some special rules apply if the "err" or "errstr" values for the
handle are already set...
If "errstr" is true then: "" [err was %s now %s]"" is appended if
$err is true and "err" is already true; "" [state was %s now %s]""
is appended if $state is true and "state" is already true; then
""\n"" and the new $errstr are appended. Obviously the %s's above
are replaced by the corresponding values.
The handle "err" value is set to $err if: $err is true; or handle
"err" value is undef; or $err is defined and the length is greater
than the handle "err" length. The effect is that an 'information'
state only overrides undef; a 'warning' overrides undef or 'infor-
mation', and an 'error' state overrides anything.
The handle "state" value is set to $state if $state is true and the
handle "err" value was set (by the rules above).
Support for warning and information states was added in DBI 1.41.
"trace"
$h->trace($trace_settings);
$h->trace($trace_settings, $trace_filename);
$trace_settings = $h->trace;
The trace() method is used to alter the trace settings for a handle
(and any future children of that handle). It can also be used to
change where the trace output is sent.
There's a similar method, "DBI->trace", which sets the global
default trace settings.
See the "TRACING" section for full details about the DBI's powerful
tracing facilities.
"trace_msg"
$h->trace_msg($message_text);
$h->trace_msg($message_text, $min_level);
Writes $message_text to the trace file if the trace level is
greater than or equal to $min_level (which defaults to 1). Can
also be called as "DBI->trace_msg($msg)".
See "TRACING" for more details.
"func"
$h->func(@func_arguments, $func_name) or die ...;
The "func" method can be used to call private non-standard and non-
portable methods implemented by the driver. Note that the function
name is given as the last argument.
It's also important to note that the func() method does not clear a
previous error ($DBI::err etc.) and it does not trigger automatic
error detection (RaiseError etc.) so you must check the return sta-
tus and/or $h->err to detect errors.
(This method is not directly related to calling stored procedures.
Calling stored procedures is currently not defined by the DBI.
Some drivers, such as DBD::Oracle, support it in non-portable ways.
See driver documentation for more details.)
See also "install_method" for how you can avoid needing to use
func() and gain.
"can"
$is_implemented = $h->can($method_name);
Returns true if $method_name is implemented by the driver or a
default method is provided by the DBI. It returns false where a
driver hasn't implemented a method and the default method is pro-
vided by the DBI is just an empty stub.
"parse_trace_flags"
$trace_settings_integer = $h->parse_trace_flags($trace_settings);
Parses a string containing trace settings and returns the corre-
sponding integer value used internally by the DBI and drivers.
The $trace_settings argument is a string containing a trace level
between 0 and 15 and/or trace flag names separated by vertical bar
(""|"") or comma ("","") characters. For example: "SQL|3|foo".
It uses the parse_trace_flag() method, described below, to process
the individual trage flag names.
The parse_trace_flags() method was added in DBI 1.42.
"parse_trace_flag"
$bit_flag = $h->parse_trace_flag($trace_flag_name);
Returns the bit flag corresponding to the trace flag name in
$trace_flag_name. Drivers are expected to override this method and
check if $trace_flag_name is a driver specific trace flags and, if
not, then call the DBIs default parse_trace_flag().
The parse_trace_flag() method was added in DBI 1.42.
"swap_inner_handle"
$rc = $h1->swap_inner_handle( $h2 );
$rc = $h1->swap_inner_handle( $h2, $allow_reparent );
Brain transplants for handles. You don't need to know about this
unless you want to become a handle surgeon.
A DBI handle is a reference to a tied hash. A tied hash has an
inner hash that actually holds the contents. The swap_inner_han-
dle() method swaps the inner hashes between two handles. The $h1
and $h2 handles still point to the same tied hashes, but what those
hashes are tied to has been swapped. In effect $h1 becomes $h2 and
vice-versa. This is powerful stuff. Use with care.
As a small safety measure, the two handles, $h1 and $h2, have to
share the same parent unless $allow_reparent is true.
The swap_inner_handle() method was added in DBI 1.44.
ATTRIBUTES COMMON TO ALL HANDLES
These attributes are common to all types of DBI handles.
Some attributes are inherited by child handles. That is, the value of
an inherited attribute in a newly created statement handle is the same
as the value in the parent database handle. Changes to attributes in
the new statement handle do not affect the parent database handle and
changes to the database handle do not affect existing statement han-
dles, only future ones.
Attempting to set or get the value of an unknown attribute generates a
warning, except for private driver specific attributes (which all have
names starting with a lowercase letter).
Example:
$h->{AttributeName} = ...; # set/write
... = $h->{AttributeName}; # get/read
"Warn" (boolean, inherited)
The "Warn" attribute enables useful warnings for certain bad prac-
tices. It is enabled by default and should only be disabled in rare
circumstances. Since warnings are generated using the Perl "warn"
function, they can be intercepted using the Perl $SIG{__WARN__}
hook.
The "Warn" attribute is not related to the "PrintWarn" attribute.
"Active" (boolean, read-only)
The "Active" attribute is true if the handle object is "active".
This is rarely used in applications. The exact meaning of active is
somewhat vague at the moment. For a database handle it typically
means that the handle is connected to a database ("$dbh->discon-
nect" sets "Active" off). For a statement handle it typically
means that the handle is a "SELECT" that may have more data to
fetch. (Fetching all the data or calling "$sth->finish" sets
"Active" off.)
"Executed" (boolean)
The "Executed" attribute is true if the handle object has been
"executed". Currently only the $dbh do() method and the $sth exe-
cute(), execute_array(), and execute_for_fetch() methods set the
"Executed" attribute.
When it's set on a handle it is also set on the parent handle at
the same time. So calling execute() on a $sth also sets the "Exe-
cuted" attribute on the parent $dbh.
The "Executed" attribute for a database handle is cleared by the
commit() and rollback() methods. The "Executed" attribute of a
statement handle is not cleared by the DBI under any circumstances
and so acts as a permanent record of whether the statement handle
was ever used.
The "Executed" attribute was added in DBI 1.41.
"Kids" (integer, read-only)
For a driver handle, "Kids" is the number of currently existing
database handles that were created from that driver handle. For a
database handle, "Kids" is the number of currently existing state-
ment handles that were created from that database handle. For a
statement handle, the value is zero.
"ActiveKids" (integer, read-only)
Like "Kids", but only counting those that are "Active" (as above).
"CachedKids" (hash ref)
For a database handle, "CachedKids" returns a reference to the
cache (hash) of statement handles created by the "prepare_cached"
method. For a driver handle, returns a reference to the cache
(hash) of database handles created by the "connect_cached" method.
"CompatMode" (boolean, inherited)
The "CompatMode" attribute is used by emulation layers (such as
Oraperl) to enable compatible behaviour in the underlying driver
(e.g., DBD::Oracle) for this handle. Not normally set by applica-
tion code.
It also has the effect of disabling the 'quick FETCH' of attribute
values from the handles attribute cache. So all attribute values
are handled by the drivers own FETCH method. This makes them
slightly slower but is useful for special-purpose drivers like
DBD::Multiplex.
"InactiveDestroy" (boolean)
The "InactiveDestroy" attribute can be used to disable the database
engine related effect of DESTROYing a handle (which would normally
close a prepared statement or disconnect from the database etc).
The default value, false, means a handle will be fully destroyed
when it passes out of scope.
For a database handle, this attribute does not disable an explicit
call to the disconnect method, only the implicit call from DESTROY
that happens if the handle is still marked as "Active".
Think of the name as meaning 'treat the handle as not-Active in the
DESTROY method'.
This attribute is specifically designed for use in Unix applica-
tions that "fork" child processes. Either the parent or the child
process, but not both, should set "InactiveDestroy" on all their
shared handles. Note that some databases, including Oracle, don't
support passing a database connection across a fork.
To help tracing applications using fork the process id is shown in
the trace log whenever a DBI or handle trace() method is called.
The process id also shown for every method call if the DBI trace
level (not handle trace level) is set high enough to show the trace
from the DBI's method dispatcher, e.g. >= 9.
"PrintWarn" (boolean, inherited)
The "PrintWarn" attribute controls the printing of warnings
recorded by the driver. When set to a true value the DBI will
check method calls to see if a warning condition has been set. If
so, the DBI will effectively do a "warn("$class $method warning:
$DBI::errstr")" where $class is the driver class and $method is the
name of the method which failed. E.g.,
DBD::Oracle::db execute warning: ... warning text here ...
By default, "DBI->connect" sets "PrintWarn" "on" if $^W is true,
i.e., perl is running with warnings enabled.
If desired, the warnings can be caught and processed using a
$SIG{__WARN__} handler or modules like CGI::Carp and CGI::Error-
Wrap.
See also "set_err" for how warnings are recorded and "HandleSetErr"
for how to influence it.
Fetching the full details of warnings can require an extra round-
trip to the database server for some drivers. In which case the
driver may opt to only fetch the full details of warnings if the
"PrintWarn" attribute is true. If "PrintWarn" is false then these
drivers should still indicate the fact that there were warnings by
setting the warning string to, for example: "3 warnings".
"PrintError" (boolean, inherited)
The "PrintError" attribute can be used to force errors to generate
warnings (using "warn") in addition to returning error codes in the
normal way. When set "on", any method which results in an error
occuring will cause the DBI to effectively do a "warn("$class
$method failed: $DBI::errstr")" where $class is the driver class
and $method is the name of the method which failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
By default, "DBI->connect" sets "PrintError" "on".
If desired, the warnings can be caught and processed using a
$SIG{__WARN__} handler or modules like CGI::Carp and CGI::Error-
Wrap.
"RaiseError" (boolean, inherited)
The "RaiseError" attribute can be used to force errors to raise
exceptions rather than simply return error codes in the normal way.
It is "off" by default. When set "on", any method which results in
an error will cause the DBI to effectively do a "die("$class
$method failed: $DBI::errstr")", where $class is the driver class
and $method is the name of the method that failed. E.g.,
DBD::Oracle::db prepare failed: ... error text here ...
If you turn "RaiseError" on then you'd normally turn "PrintError"
off. If "PrintError" is also on, then the "PrintError" is done
first (naturally).
Typically "RaiseError" is used in conjunction with "eval { ... }"
to catch the exception that's been thrown and followed by an "if
($@) { ... }" block to handle the caught exception. In that eval
block the $DBI::lasth variable can be useful for diagnosis and
reporting. For example, $DBI::lasth->{Type} and
$DBI::lasth->{Statement}.
If you want to temporarily turn "RaiseError" off (inside a library
function that is likely to fail, for example), the recommended way
is like this:
{
local $h->{RaiseError}; # localize and turn off for this block
...
}
The original value will automatically and reliably be restored by
Perl, regardless of how the block is exited. The same logic
applies to other attributes, including "PrintError".
"HandleError" (code ref, inherited)
The "HandleError" attribute can be used to provide your own alter-
native behaviour in case of errors. If set to a reference to a sub-
routine then that subroutine is called when an error is detected
(at the same point that "RaiseError" and "PrintError" are handled).
The subroutine is called with three parameters: the error message
string that "RaiseError" and "PrintError" would use, the DBI handle
being used, and the first value being returned by the method that
failed (typically undef).
If the subroutine returns a false value then the "RaiseError"
and/or "PrintError" attributes are checked and acted upon as nor-
mal.
For example, to "die" with a full stack trace for any error:
use Carp;
$h->{HandleError} = sub { confess(shift) };
Or to turn errors into exceptions:
use Exception; # or your own favourite exception module
$h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };
It is possible to 'stack' multiple HandleError handlers by using
closures:
sub your_subroutine {
my $previous_handler = $h->{HandleError};
$h->{HandleError} = sub {
return 1 if $previous_handler and &$previous_handler(@_);
... your code here ...
};
}
Using a "my" inside a subroutine to store the previous "HandleEr-
ror" value is important. See perlsub and perlref for more informa-
tion about closures.
It is possible for "HandleError" to alter the error message that
will be used by "RaiseError" and "PrintError" if it returns false.
It can do that by altering the value of $_[0]. This example appends
a stack trace to all errors and, unlike the previous example using
Carp::confess, this will work "PrintError" as well as "RaiseError":
$h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };
It is also possible for "HandleError" to hide an error, to a lim-
ited degree, by using "set_err" to reset $DBI::err and
$DBI::errstr, and altering the return value of the failed method.
For example:
$h->{HandleError} = sub {
return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
return 0 unless $_[1]->err == 1234; # the error to 'hide'
$h->set_err(undef,undef); # turn off the error
$_[2] = [ ... ]; # supply alternative return value
return 1;
};
This only works for methods which return a single value and is hard
to make reliable (avoiding infinite loops, for example) and so
isn't recommended for general use! If you find a good use for it
then please let me know.
"HandleSetErr" (code ref, inherited)
The "HandleSetErr" attribute can be used to intercept the setting
of handle "err", "errstr", and "state" values. If set to a refer-
ence to a subroutine then that subroutine is called whenever
set_err() is called, typically by the driver or a subclass.
The subroutine is called with five arguments, the first five that
were passed to set_err(): the handle, the "err", "errstr", and
"state" values being set, and the method name. These can be altered
by changing the values in the @_ array. The return value affects
set_err() behaviour, see "set_err" for details.
It is possible to 'stack' multiple HandleSetErr handlers by using
closures. See "HandleError" for an example.
The "HandleSetErr" and "HandleError" subroutines differ in subtle
but significant ways. HandleError is only invoked at the point
where the DBI is about to return to the application with "err" set
true. It's not invoked by the failure of a method that's been
called by another DBI method. HandleSetErr, on the other hand, is
called whenever set_err() is called with a defined "err" value,
even if false. So it's not just for errors, despite the name, but
also warn and info states. The set_err() method, and thus Handle-
SetErr, may be called multiple times within a method and is usually
invoked from deep within driver code.
In theory a driver can use the return value from HandleSetErr via
set_err() to decide whether to continue or not. If set_err()
returns an empty list, indicating that the HandleSetErr code has
'handled' the 'error', the driver could then continue instead of
failing (if that's a reasonable thing to do). This isn't excepted
to be common and any such cases should be clearly marked in the
driver documentation and discussed on the dbi-dev mailing list.
The "HandleSetErr" attribute was added in DBI 1.41.
"ErrCount" (unsigned integer)
The "ErrCount" attribute is incremented whenever the set_err()
method records an error. It isn't incremented by warnings or infor-
mation states. It is not reset by the DBI at any time.
The "ErrCount" attribute was added in DBI 1.41. Older drivers may
not have been updated to use set_err() to record errors and so this
attribute may not be incremented when using them.
"ShowErrorStatement" (boolean, inherited)
The "ShowErrorStatement" attribute can be used to cause the rele-
vant Statement text to be appended to the error messages generated
by the "RaiseError", "PrintError", and "PrintWarn" attributes.
Only applies to errors on statement handles plus the prepare(),
do(), and the various "select*()" database handle methods. (The
exact format of the appended text is subject to change.)
If "$h->{ParamValues}" returns a hash reference of parameter
(placeholder) values then those are formatted and appended to the
end of the Statement text in the error message.
"TraceLevel" (integer, inherited)
The "TraceLevel" attribute can be used as an alternative to the
"trace" method to set the DBI trace level and trace flags for a
specific handle. See "TRACING" for more details.
The "TraceLevel" attribute is especially useful combined with
"local" to alter the trace settings for just a single block of
code.
"FetchHashKeyName" (string, inherited)
The "FetchHashKeyName" attribute is used to specify whether the
fetchrow_hashref() method should perform case conversion on the
field names used for the hash keys. For historical reasons it
defaults to '"NAME"' but it is recommended to set it to '"NAME_lc"'
(convert to lower case) or '"NAME_uc"' (convert to upper case)
according to your preference. It can only be set for driver and
database handles. For statement handles the value is frozen when
prepare() is called.
"ChopBlanks" (boolean, inherited)
The "ChopBlanks" attribute can be used to control the trimming of
trailing space characters from fixed width character (CHAR) fields.
No other field types are affected, even where field values have
trailing spaces.
The default is false (although it is possible that the default may
change). Applications that need specific behaviour should set the
attribute as needed.
Drivers are not required to support this attribute, but any driver
which does not support it must arrange to return "undef" as the
attribute value.
"LongReadLen" (unsigned integer, inherited)
The "LongReadLen" attribute may be used to control the maximum
length of 'long' fields ("blob", "memo", etc.) which the driver
will read from the database automatically when it fetches each row
of data.
The "LongReadLen" attribute only relates to fetching and reading
long values; it is not involved in inserting or updating them.
A value of 0 means not to automatically fetch any long data.
("fetch" should return "undef" for long fields when "LongReadLen"
is 0.)
The default is typically 0 (zero) bytes but may vary between
drivers. Applications fetching long fields should set this value
to slightly larger than the longest long field value to be fetched.
Some databases return some long types encoded as pairs of hex dig-
its. For these types, "LongReadLen" relates to the underlying data
length and not the doubled-up length of the encoded string.
Changing the value of "LongReadLen" for a statement handle after it
has been "prepare"'d will typically have no effect, so it's common
to set "LongReadLen" on the $dbh before calling "prepare".
For most drivers the value used here has a direct effect on the
memory used by the statement handle while it's active, so don't be
too generous. If you can't be sure what value to use you could exe-
cute an extra select statement to determine the longest value. For
example:
$dbh->{LongReadLen} = $dbh->selectrow_array{qq{
SELECT MAX(long_column_name) FROM table WHERE ...
});
$sth = $dbh->prepare(qq{
SELECT long_column_name, ... FROM table WHERE ...
});
You may need to take extra care if the table can be modified
between the first select and the second being executed.
See "LongTruncOk" for more information on truncation behaviour.
"LongTruncOk" (boolean, inherited)
The "LongTruncOk" attribute may be used to control the effect of
fetching a long field value which has been truncated (typically
because it's longer than the value of the "LongReadLen" attribute).
By default, "LongTruncOk" is false and so fetching a long value
that needs to be truncated will cause the fetch to fail. (Applica-
tions should always be sure to check for errors after a fetch loop
in case an error, such as a divide by zero or long field trunca-
tion, caused the fetch to terminate prematurely.)
If a fetch fails due to a long field truncation when "LongTruncOk"
is false, many drivers will allow you to continue fetching further
rows.
See also "LongReadLen".
"TaintIn" (boolean, inherited)
If the "TaintIn" attribute is set to a true value and Perl is run-
ning in taint mode (e.g., started with the "-T" option), then all
the arguments to most DBI method calls are checked for being
tainted. This may change.
The attribute defaults to off, even if Perl is in taint mode. See
perlsec for more about taint mode. If Perl is not running in taint
mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintIn
attribute, for that statement handle, for the duration of the fetch
loop.
The "TaintIn" attribute was added in DBI 1.31.
"TaintOut" (boolean, inherited)
If the "TaintOut" attribute is set to a true value and Perl is run-
ning in taint mode (e.g., started with the "-T" option), then most
data fetched from the database is considered tainted. This may
change.
The attribute defaults to off, even if Perl is in taint mode. See
perlsec for more about taint mode. If Perl is not running in taint
mode, this attribute has no effect.
When fetching data that you trust you can turn off the TaintOut
attribute, for that statement handle, for the duration of the fetch
loop.
Currently only fetched data is tainted. It is possible that the
results of other DBI method calls, and the value of fetched
attributes, may also be tainted in future versions. That change may
well break your applications unless you take great care now. If you
use DBI Taint mode, please report your experience and any sugges-
tions for changes.
The "TaintOut" attribute was added in DBI 1.31.
"Taint" (boolean, inherited)
The "Taint" attribute is a shortcut for "TaintIn" and "TaintOut"
(it is also present for backwards compatibility).
Setting this attribute sets both "TaintIn" and "TaintOut", and
retrieving it returns a true value if and only if "TaintIn" and
"TaintOut" are both set to true values.
"Profile" (inherited)
The "Profile" attribute enables the collection and reporting of
method call timing statistics. See the DBI::Profile module docu-
mentation for much more detail.
The "Profile" attribute was added in DBI 1.24.
"private_your_module_name_*"
The DBI provides a way to store extra information in a DBI handle
as "private" attributes. The DBI will allow you to store and
retrieve any attribute which has a name starting with ""private_"".
It is strongly recommended that you use just one private attribute
(e.g., use a hash ref) and give it a long and unambiguous name that
includes the module or application name that the attribute relates
to (e.g., ""private_YourFullModuleName_thingy"").
Because of the way the Perl tie mechanism works you cannot reliably
use the "||=" operator directly to initialise the attribute, like
this:
my $foo = $dbh->{private_yourmodname_foo} ||= { ... }; # WRONG
you should use a two step approach like this:
my $foo = $dbh->{private_yourmodname_foo};
$foo ||= $dbh->{private_yourmodname_foo} = { ... };
This attribute is primarily of interest to people sub-classing DBI.
DBI DATABASE HANDLE OBJECTS
This section covers the methods and attributes associated with database
handles.
Database Handle Methods
The following methods are specified for DBI database handles:
"clone"
$new_dbh = $dbh->clone();
$new_dbh = $dbh->clone(\%attr);
The "clone" method duplicates the $dbh connection by connecting
with the same parameters ($dsn, $user, $password) as originally
used.
The attributes for the cloned connect are the same as those used
for the original connect, with some other attribute merged over
them depending on the \%attr parameter.
If \%attr is given then the attributes it contains are merged into
the original attributes and override any with the same names.
Effectively the same as doing:
%attribues_used = ( %original_attributes, %attr );
If \%attr is not given then it defaults to a hash containing all
the attributes in the attribute cache of $dbh excluding any non-
code references, plus the main boolean attributes (RaiseError,
PrintError, AutoCommit, etc.). This behaviour is subject to change.
The clone method can be used even if the database handle is discon-
nected.
The "clone" method was added in DBI 1.33. It is very new and likely
to change.
"data_sources"
@ary = $dbh->data_sources();
@ary = $dbh->data_sources(\%attr);
Returns a list of data sources (databases) available via the $dbh
driver's data_sources() method, plus any extra data sources that
the driver can discover via the connected $dbh. Typically the extra
data sources are other databases managed by the same server process
that the $dbh is connected to.
Data sources are returned in a form suitable for passing to the
"connect" method (that is, they will include the ""dbi:$driver:""
prefix).
The data_sources() method, for a $dbh, was added in DBI 1.38.
"do"
$rows = $dbh->do($statement) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr) or die $dbh->errstr;
$rows = $dbh->do($statement, \%attr, @bind_values) or die ...
Prepare and execute a single statement. Returns the number of rows
affected or "undef" on error. A return value of "-1" means the num-
ber of rows is not known, not applicable, or not available.
This method is typically most useful for non-"SELECT" statements
that either cannot be prepared in advance (due to a limitation of
the driver) or do not need to be executed repeatedly. It should not
be used for "SELECT" statements because it does not return a state-
ment handle (so you can't fetch any data).
The default "do" method is logically similar to:
sub do {
my($dbh, $statement, $attr, @bind_values) = @_;
my $sth = $dbh->prepare($statement, $attr) or return undef;
$sth->execute(@bind_values) or return undef;
my $rows = $sth->rows;
($rows == 0) ? "0E0" : $rows; # always return true if no error
}
For example:
my $rows_deleted = $dbh->do(q{
DELETE FROM table
WHERE status = ?
}, undef, 'DONE') or die $dbh->errstr;
Using placeholders and @bind_values with the "do" method can be
useful because it avoids the need to correctly quote any variables
in the $statement. But if you'll be executing the statement many
times then it's more efficient to "prepare" it once and call "exe-
cute" many times instead.
The "q{...}" style quoting used in this example avoids clashing
with quotes that may be used in the SQL statement. Use the double-
quote-like "qq{...}" operator if you want to interpolate variables
into the string. See "Quote and Quote-like Operators" in perlop
for more details.
"last_insert_id"
$rv = $dbh->last_insert_id($catalog, $schema, $table, $field);
$rv = $dbh->last_insert_id($catalog, $schema, $table, $field, \%attr
);
Returns a value 'identifying' the row just inserted, if possible.
Typically this would be a value assigned by the database server to
a column with an auto_increment or serial type. Returns undef if
the driver does not support the method or can't determine the
value.
The $catalog, $schema, $table, and $field parameters may be
required for some drivers (see below). If you don't know the
parameter values and your driver does not need them, then use
"undef" for each.
There are several caveats to be aware of with this method if you
want to use it for portable applications:
* For some drivers the value may only available immediately after
the insert statement has executed (e.g., mysql, Informix).
* For some drivers the $catalog, $schema, $table, and $field param-
eters are required (e.g., Pg), for others they are ignored (e.g.,
mysql).
* Drivers may return an indeterminate value if no insert has been
performed yet.
* For some drivers the value may only be available if placeholders
have not been used (e.g., Sybase, MS SQL). In this case the value
returned would be from the last non-placeholder insert statement.
* Some drivers may need driver-specific hints about how to get the
value. For example, being told the name of the database 'sequence'
object that holds the value. Any such hints are passed as driver-
specific attributes in the \%attr parameter.
* If the underlying database offers nothing better, then some
drivers may attempt to implement this method by executing ""select
max($field) from $table"". Drivers using any approach like this
should issue a warning if "AutoCommit" is true because it is gener-
ally unsafe - another process may have modified the table between
your insert and the select. For situations where you know it is
safe, such as when you have locked the table, you can silence the
warning by passing "Warn" => 0 in \%attr.
* If no insert has been performed yet, or the last insert failed,
then the value is implementation defined.
Given all the caveats above, it's clear that this method must be
used with care.
The "last_insert_id" method was added in DBI 1.38.
"selectrow_array"
@row_ary = $dbh->selectrow_array($statement);
@row_ary = $dbh->selectrow_array($statement, \%attr);
@row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_array" into a single call. If called in a list context,
it returns the first row of data from the statement. The $state-
ment parameter can be a previously prepared statement handle, in
which case the "prepare" is skipped.
If any method fails, and "RaiseError" is not set, "selectrow_array"
will return an empty list.
If called in a scalar context for a statement handle that has more
than one column, it is undefined whether the driver will return the
value of the first column or the last. So don't do that. Also, in
a scalar context, an "undef" is returned if there are no more rows
or if an error occurred. That "undef" can't be distinguished from
an "undef" returned because the first field value was NULL. For
these reasons you should exercise some caution if you use "selec-
trow_array" in a scalar context.
"selectrow_arrayref"
$ary_ref = $dbh->selectrow_arrayref($statement);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
$ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_arrayref" into a single call. It returns the first row of
data from the statement. The $statement parameter can be a
previously prepared statement handle, in which case the "prepare"
is skipped.
If any method fails, and "RaiseError" is not set, "selectrow_array"
will return undef.
"selectrow_hashref"
$hash_ref = $dbh->selectrow_hashref($statement);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr);
$hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);
This utility method combines "prepare", "execute" and
"fetchrow_hashref" into a single call. It returns the first row of
data from the statement. The $statement parameter can be a previ-
ously prepared statement handle, in which case the "prepare" is
skipped.
If any method fails, and "RaiseError" is not set, "selec-
trow_hashref" will return undef.
"selectall_arrayref"
$ary_ref = $dbh->selectall_arrayref($statement);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr);
$ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values
);
This utility method combines "prepare", "execute" and
"fetchall_arrayref" into a single call. It returns a reference to
an array containing a reference to an array for each row of data
fetched.
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If "RaiseError" is not set and any method except
"fetchall_arrayref" fails then "selectall_arrayref" will return
"undef"; if "fetchall_arrayref" fails then it will return with
whatever data has been fetched thus far. You should check
"$sth->err" afterwards (or use the "RaiseError" attribute) to dis-
cover if the data is complete or was truncated due to an error.
The "fetchall_arrayref" method called by "selectall_arrayref" sup-
ports a $max_rows parameter. You can specify a value for $max_rows
by including a '"MaxRows"' attribute in \%attr. In which case fin-
ish() is called for you after fetchall_arrayref() returns.
The "fetchall_arrayref" method called by "selectall_arrayref" also
supports a $slice parameter. You can specify a value for $slice by
including a '"Slice"' or '"Columns"' attribute in \%attr. The only
difference between the two is that if "Slice" is not defined and
"Columns" is an array ref, then the array is assumed to contain
column index values (which count from 1), rather than perl array
index values. In which case the array is copied and each value
decremented before passing to "/fetchall_arrayref".
"selectall_hashref"
$hash_ref = $dbh->selectall_hashref($statement, $key_field);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
$hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr,
@bind_values);
This utility method combines "prepare", "execute" and
"fetchall_hashref" into a single call. It returns a reference to a
hash containing one entry for each row. The key for each row entry
is specified by $key_field. The value is a reference to a hash
returned by "fetchrow_hashref".
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If any method except "fetchrow_hashref" fails, and "RaiseError" is
not set, "selectall_hashref" will return "undef". If
"fetchrow_hashref" fails and "RaiseError" is not set, then it will
return with whatever data it has fetched thus far. $DBI::err should
be checked to catch that.
"selectcol_arrayref"
$ary_ref = $dbh->selectcol_arrayref($statement);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
$ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values
);
This utility method combines "prepare", "execute", and fetching one
column from all the rows, into a single call. It returns a refer-
ence to an array containing the values of the first column from
each row.
The $statement parameter can be a previously prepared statement
handle, in which case the "prepare" is skipped. This is recommended
if the statement is going to be executed many times.
If any method except "fetch" fails, and "RaiseError" is not set,
"selectcol_arrayref" will return "undef". If "fetch" fails and
"RaiseError" is not set, then it will return with whatever data it
has fetched thus far. $DBI::err should be checked to catch that.
The "selectcol_arrayref" method defaults to pushing a single column
value (the first) from each row into the result array. However, it
can also push another column, or even multiple columns per row,
into the result array. This behaviour can be specified via a
'"Columns"' attribute which must be a ref to an array containing
the column number or numbers to use. For example:
# get array of id and name pairs:
my $ary_ref = $dbh->selectcol_arrayref("select id, name from table",
{ Columns=>[1,2] });
my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id
} => name
You can specify a maximum number of rows to fetch by including a
'"MaxRows"' attribute in \%attr.
"prepare"
$sth = $dbh->prepare($statement) or die $dbh->errstr;
$sth = $dbh->prepare($statement, \%attr) or die $dbh->errstr;
Prepares a statement for later execution by the database engine and
returns a reference to a statement handle object.
The returned statement handle can be used to get attributes of the
statement and invoke the "execute" method. See "Statement Handle
Methods".
Drivers for engines without the concept of preparing a statement
will typically just store the statement in the returned handle and
process it when "$sth->execute" is called. Such drivers are
unlikely to give much useful information about the statement, such
as "$sth->{NUM_OF_FIELDS}", until after "$sth->execute" has been
called. Portable applications should take this into account.
In general, DBI drivers do not parse the contents of the statement
(other than simply counting any "Placeholders"). The statement is
passed directly to the database engine, sometimes known as pass-
thru mode. This has advantages and disadvantages. On the plus side,
you can access all the functionality of the engine being used. On
the downside, you're limited if you're using a simple engine, and
you need to take extra care if writing applications intended to be
portable between engines.
Portable applications should not assume that a new statement can be
prepared and/or executed while still fetching results from a previ-
ous statement.
Some command-line SQL tools use statement terminators, like a semi-
colon, to indicate the end of a statement. Such terminators should
not normally be used with the DBI.
"prepare_cached"
$sth = $dbh->prepare_cached($statement)
$sth = $dbh->prepare_cached($statement, \%attr)
$sth = $dbh->prepare_cached($statement, \%attr, $if_active)
Like "prepare" except that the statement handle returned will be
stored in a hash associated with the $dbh. If another call is made
to "prepare_cached" with the same $statement and %attr parameter
values, then the corresponding cached $sth will be returned without
contacting the database server.
The $if_active parameter lets you adjust the behaviour if an
already cached statement handle is still Active. There are several
alternatives:
0: A warning will be generated, and finish() will be called on
the statement handle before it is returned. This is the
default behaviour if $if_active is not passed.
1: finish() will be called on the statement handle, but the
warning is suppressed.
2: Disables any checking.
3: The existing active statement handle will be removed from
the cache and a new statement handle prepared and cached in its
place. This is the safest option because it doesn't affect the
state of the old handle, it just removes it from the cache.
[Added in DBI 1.40]
Here are some examples of "prepare_cached":
sub insert_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $sql = sprintf "insert into %s (%s) values (%s)",
$table, join(",", @fields), join(",", ("?")x@fields);
my $sth = $dbh->prepare_cached($sql);
return $sth->execute(@values);
}
sub search_hash {
my ($table, $field_values) = @_;
my @fields = sort keys %$field_values; # sort required
my @values = @{$field_values}{@fields};
my $qualifier = "";
$qualifier = "where ".join(" and ", map { "$_=?" } @fields) if @fi
elds;
$sth = $dbh->prepare_cached("SELECT * FROM $table $qualifier");
return $dbh->selectall_arrayref($sth, {}, @values);
}
Caveat emptor: This caching can be useful in some applications, but
it can also cause problems and should be used with care. Here is a
contrived case where caching would cause a significant problem:
my $sth = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth->execute(...);
while (my $data = $sth->fetchrow_hashref) {
# later, in some other code called within the loop...
my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
$sth2->execute(...);
while (my $data2 = $sth2->fetchrow_arrayref) {
do_stuff(...);
}
}
In this example, since both handles are preparing the exact same
statement, $sth2 will not be its own statement handle, but a dupli-
cate of $sth returned from the cache. The results will certainly
not be what you expect. Typically the the inner fetch loop will
work normally, fetching all the records and terminating when there
are no more, but now $sth is the same as $sth2 the outer fetch loop
will also terminate.
You'll know if you run into this problem because prepare_cached()
will generate a warning by default (when $if_active is false).
The cache used by prepare_cached() is keyed by both the statement
and any attributes so you can also avoid this issue by doing some-
thing like:
$sth = $dbh->prepare_cached("...", { dbi_dummy => __FILE__.__LINE__
});
which will ensure that prepare_cached only returns statements
cached by that line of code in that source file.
"commit"
$rc = $dbh->commit or die $dbh->errstr;
Commit (make permanent) the most recent series of database changes
if the database supports transactions and AutoCommit is off.
If "AutoCommit" is on, then calling "commit" will issue a "commit
ineffective with AutoCommit" warning.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"rollback"
$rc = $dbh->rollback or die $dbh->errstr;
Rollback (undo) the most recent series of uncommitted database
changes if the database supports transactions and AutoCommit is
off.
If "AutoCommit" is on, then calling "rollback" will issue a "roll-
back ineffective with AutoCommit" warning.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"begin_work"
$rc = $dbh->begin_work or die $dbh->errstr;
Enable transactions (by turning "AutoCommit" off) until the next
call to "commit" or "rollback". After the next "commit" or "roll-
back", "AutoCommit" will automatically be turned on again.
If "AutoCommit" is already off when "begin_work" is called then it
does nothing except return an error. If the driver does not support
transactions then when "begin_work" attempts to set "AutoCommit"
off the driver will trigger a fatal error.
See also "Transactions" in the "FURTHER INFORMATION" section below.
"disconnect"
$rc = $dbh->disconnect or warn $dbh->errstr;
Disconnects the database from the database handle. "disconnect" is
typically only used before exiting the program. The handle is of
little use after disconnecting.
The transaction behaviour of the "disconnect" method is, sadly,
undefined. Some database systems (such as Oracle and Ingres) will
automatically commit any outstanding changes, but others (such as
Informix) will rollback any outstanding changes. Applications not
using "AutoCommit" should explicitly call "commit" or "rollback"
before calling "disconnect".
The database is automatically disconnected by the "DESTROY" method
if still connected when there are no longer any references to the
handle. The "DESTROY" method for each driver should implicitly
call "rollback" to undo any uncommitted changes. This is vital
behaviour to ensure that incomplete transactions don't get commit-
ted simply because Perl calls "DESTROY" on every object before
exiting. Also, do not rely on the order of object destruction dur-
ing "global destruction", as it is undefined.
Generally, if you want your changes to be commited or rolled back
when you disconnect, then you should explicitly call "commit" or
"rollback" before disconnecting.
If you disconnect from a database while you still have active
statement handles (e.g., SELECT statement handles that may have
more data to fetch), you will get a warning. The warning may indi-
cate that a fetch loop terminated early, perhaps due to an uncaught
error. To avoid the warning call the "finish" method on the active
handles.
"ping"
$rc = $dbh->ping;
Attempts to determine, in a reasonably efficient way, if the
database server is still running and the connection to it is still
working. Individual drivers should implement this function in the
most suitable manner for their database engine.
The current default implementation always returns true without
actually doing anything. Actually, it returns ""0 but true"" which
is true but zero. That way you can tell if the return value is gen-
uine or just the default. Drivers should override this method with
one that does the right thing for their type of database.
Few applications would have direct use for this method. See the
specialized Apache::DBI module for one example usage.
"get_info"
$value = $dbh->get_info( $info_type );
Returns information about the implementation, i.e. driver and data
source capabilities, restrictions etc. It returns "undef" for
unknown or unimplemented information types. For example:
$database_version = $dbh->get_info( 18 ); # SQL_DBMS_VER
$max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_
SELECT
See "Standards Reference Information" for more detailed information
about the information types and their meanings and possible return
values.
The DBI::Const::GetInfoType module exports a %GetInfoType hash that
can be used to map info type names to numbers. For example:
$database_version = $dbh->get_info( $GetInfoType{SQL_DBMS_VER} );
The names are a merging of the ANSI and ODBC standards (which dif-
fer in some cases). See DBI::Const::GetInfoType for more details.
Because some DBI methods make use of get_info(), drivers are
strongly encouraged to support at least the following very minimal
set of information types to ensure the DBI itself works properly:
Type Name Example A Example B
---- -------------------------- ------------ ----------------
17 SQL_DBMS_NAME 'ACCESS' 'Oracle'
18 SQL_DBMS_VER '03.50.0000' '08.01.0721 ...'
29 SQL_IDENTIFIER_QUOTE_CHAR '`' '"'
41 SQL_CATALOG_NAME_SEPARATOR '.' '@'
114 SQL_CATALOG_LOCATION 1 2
"table_info"
$sth = $dbh->table_info( $catalog, $schema, $table, $type );
$sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );
Returns an active statement handle that can be used to fetch infor-
mation about tables and views that exist in the database.
The arguments $catalog, $schema and $table may accept search pat-
terns according to the database/driver, for example: $table =
'%FOO%'; Remember that the underscore character ('"_"') is a search
pattern that means match any character, so 'FOO_%' is the same as
'FOO%' and 'FOO_BAR%' will match names like 'FOO1BAR'.
The value of $type is a comma-separated list of one or more types
of tables to be returned in the result set. Each value may option-
ally be quoted, e.g.:
$type = "TABLE";
$type = "'TABLE','VIEW'";
In addition the following special cases may also be supported by
some drivers:
* If the value of $catalog is '%' and $schema and $table name are
empty strings, the result set contains a list of catalog names. For
example:
$sth = $dbh->table_info('%', , );
* If the value of $schema is '%' and $catalog and $table are empty
strings, the result set contains a list of schema names.
* If the value of $type is '%' and $catalog, $schema, and $table
are all empty strings, the result set contains a list of table
types.
If your driver doesn't support one or more of the selection filter
parameters then you may get back more than you asked for and can do
the filtering yourself.
This method can be expensive, and can return a large amount of
data. (For example, small Oracle installation returns over 2000
rows.) So it's a good idea to use the filters to limit the data as
much as possible.
The statement handle returned has at least the following fields in
the order show below. Other fields, after these, may also be
present.
TABLE_CAT: Table catalog identifier. This field is NULL ("undef")
if not applicable to the data source, which is usually the case.
This field is empty if not applicable to the table.
TABLE_SCHEM: The name of the schema containing the TABLE_NAME
value. This field is NULL ("undef") if not applicable to data
source, and empty if not applicable to the table.
TABLE_NAME: Name of the table (or view, synonym, etc).
TABLE_TYPE: One of the following: "TABLE", "VIEW", "SYSTEM TABLE",
"GLOBAL TEMPORARY", "LOCAL TEMPORARY", "ALIAS", "SYNONYM" or a type
identifier that is specific to the data source.
REMARKS: A description of the table. May be NULL ("undef").
Note that "table_info" might not return records for all tables.
Applications can use any valid table regardless of whether it's
returned by "table_info".
See also "tables", "Catalog Methods" and "Standards Reference
Information".
"column_info"
$sth = $dbh->column_info( $catalog, $schema, $table, $column );
Returns an active statement handle that can be used to fetch infor-
mation about columns in specified tables.
The arguments $schema, $table and $column may accept search pat-
terns according to the database/driver, for example: $table =
'%FOO%';
Note: The support for the selection criteria is driver specific. If
the driver doesn't support one or more of them then you may get
back more than you asked for and can do the filtering yourself.
The statement handle returned has at least the following fields in
the order shown below. Other fields, after these, may also be
present.
TABLE_CAT: The catalog identifier. This field is NULL ("undef") if
not applicable to the data source, which is often the case. This
field is empty if not applicable to the table.
TABLE_SCHEM: The schema identifier. This field is NULL ("undef")
if not applicable to the data source, and empty if not applicable
to the table.
TABLE_NAME: The table identifier. Note: A driver may provide col-
umn metadata not only for base tables, but also for derived objects
like SYNONYMS etc.
COLUMN_NAME: The column identifier.
DATA_TYPE: The concise data type code.
TYPE_NAME: A data source dependent data type name.
COLUMN_SIZE: The column size. This is the maximum length in char-
acters for character data types, the number of digits or bits for
numeric data types or the length in the representation of temporal
types. See the relevant specifications for detailed information.
BUFFER_LENGTH: The length in bytes of transferred data.
DECIMAL_DIGITS: The total number of significant digits to the right
of the decimal point.
NUM_PREC_RADIX: The radix for numeric precision. The value is 10
or 2 for numeric data types and NULL ("undef") if not applicable.
NULLABLE: Indicates if a column can accept NULLs. The following
values are defined:
SQL_NO_NULLS 0
SQL_NULLABLE 1
SQL_NULLABLE_UNKNOWN 2
REMARKS: A description of the column.
COLUMN_DEF: The default value of the column.
SQL_DATA_TYPE: The SQL data type.
SQL_DATETIME_SUB: The subtype code for datetime and interval data
types.
CHAR_OCTET_LENGTH: The maximum length in bytes of a character or
binary data type column.
ORDINAL_POSITION: The column sequence number (starting with 1).
IS_NULLABLE: Indicates if the column can accept NULLs. Possible
values are: 'NO', 'YES' and .
SQL/CLI defines the following additional columns:
CHAR_SET_CAT
CHAR_SET_SCHEM
CHAR_SET_NAME
COLLATION_CAT
COLLATION_SCHEM
COLLATION_NAME
UDT_CAT
UDT_SCHEM
UDT_NAME
DOMAIN_CAT
DOMAIN_SCHEM
DOMAIN_NAME
SCOPE_CAT
SCOPE_SCHEM
SCOPE_NAME
MAX_CARDINALITY
DTD_IDENTIFIER
IS_SELF_REF
Drivers capable of supplying any of those values should do so in
the corresponding column and supply undef values for the others.
Drivers wishing to provide extra database/driver specific informa-
tion should do so in extra columns beyond all those listed above,
and use lowercase field names with the driver-specific prefix
(i.e., 'ora_...'). Applications accessing such fields should do so
by name and not by column number.
The result set is ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME and
ORDINAL_POSITION.
Note: There is some overlap with statement attributes (in perl) and
SQLDescribeCol (in ODBC). However, SQLColumns provides more meta-
data.
See also "Catalog Methods" and "Standards Reference Information".
"primary_key_info"
$sth = $dbh->primary_key_info( $catalog, $schema, $table );
Returns an active statement handle that can be used to fetch infor-
mation about columns that make up the primary key for a table. The
arguments don't accept search patterns (unlike table_info()).
For example:
$sth = $dbh->primary_key_info( undef, $user, 'foo' );
$data = $sth->fetchall_arrayref;
The statement handle will return one row per column, ordered by TA-
BLE_CAT, TABLE_SCHEM, TABLE_NAME, and KEY_SEQ.
Note: The support for the selection criteria, such as $catalog, is
driver specific. If the driver doesn't support catalogs and/or
schemas, it may ignore these criteria.
The statement handle returned has at least the following fields in
the order shown below. Other fields, after these, may also be
present.
TABLE_CAT: The catalog identifier. This field is NULL ("undef") if
not applicable to the data source, which is often the case. This
field is empty if not applicable to the table.
TABLE_SCHEM: The schema identifier. This field is NULL ("undef")
if not applicable to the data source, and empty if not applicable
to the table.
TABLE_NAME: The table identifier.
COLUMN_NAME: The column identifier.
KEY_SEQ: The column sequence number (starting with 1). Note: This
field is named ORDINAL_POSITION in SQL/CLI.
PK_NAME: The primary key constraint identifier. This field is NULL
("undef") if not applicable to the data source.
See also "Catalog Methods" and "Standards Reference Information".
"primary_key"
@key_column_names = $dbh->primary_key( $catalog, $schema, $table );
Simple interface to the primary_key_info() method. Returns a list
of the column names that comprise the primary key of the specified
table. The list is in primary key column sequence order.
"foreign_key_info"
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table );
$sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
, $fk_catalog, $fk_schema, $fk_table
, \%attr );
Returns an active statement handle that can be used to fetch infor-
mation about foreign keys in and/or referencing the specified ta-
ble(s). The arguments don't accept search patterns (unlike ta-
ble_info()).
$pk_catalog, $pk_schema, $pk_table identify the primary (unique)
key table (PKT).
$fk_catalog, $fk_schema, $fk_table identify the foreign key table
(FKT).
If both PKT and FKT are given, the function returns the foreign
key, if any, in table FKT that refers to the primary (unique) key
of table PKT. (Note: In SQL/CLI, the result is implementa-
tion-defined.)
If only PKT is given, then the result set contains the primary key
of that table and all foreign keys that refer to it.
If only FKT is given, then the result set contains all foreign keys
in that table and the primary keys to which they refer. (Note: In
SQL/CLI, the result includes unique keys too.)
For example:
$sth = $dbh->foreign_key_info( undef, $user, 'master');
$sth = $dbh->foreign_key_info( undef, undef, undef , undef, $user,
'detail');
$sth = $dbh->foreign_key_info( undef, $user, 'master', undef, $user,
'detail');
Note: The support for the selection criteria, such as $catalog, is
driver specific. If the driver doesn't support catalogs and/or
schemas, it may ignore these criteria.
The statement handle returned has the following fields in the order
shown below. Because ODBC never includes unique keys, they define
different columns in the result set than SQL/CLI. SQL/CLI column
names are shown in parentheses.
PKTABLE_CAT ( UK_TABLE_CAT ): The primary (unique) key ta-
ble catalog identifier. This field is NULL ("undef") if not appli-
cable to the data source, which is often the case. This field is
empty if not applicable to the table.
PKTABLE_SCHEM ( UK_TABLE_SCHEM ): The primary (unique) key ta-
ble schema identifier. This field is NULL ("undef") if not appli-
cable to the data source, and empty if not applicable to the table.
PKTABLE_NAME ( UK_TABLE_NAME ): The primary (unique) key ta-
ble identifier.
PKCOLUMN_NAME (UK_COLUMN_NAME ): The primary (unique) key col-
umn identifier.
FKTABLE_CAT ( FK_TABLE_CAT ): The foreign key table catalog
identifier. This field is NULL ("undef") if not applicable to the
data source, which is often the case. This field is empty if not
applicable to the table.
FKTABLE_SCHEM ( FK_TABLE_SCHEM ): The foreign key table schema
identifier. This field is NULL ("undef") if not applicable to the
data source, and empty if not applicable to the table.
FKTABLE_NAME ( FK_TABLE_NAME ): The foreign key table identi-
fier.
FKCOLUMN_NAME ( FK_COLUMN_NAME ): The foreign key column iden-
tifier.
KEY_SEQ ( ORDINAL_POSITION ): The column sequence number
(starting with 1).
UPDATE_RULE ( UPDATE_RULE ): The referential action for
the UPDATE rule. The following codes are defined:
CASCADE 0
RESTRICT 1
SET NULL 2
NO ACTION 3
SET DEFAULT 4
DELETE_RULE ( DELETE_RULE ): The referential action for
the DELETE rule. The codes are the same as for UPDATE_RULE.
FK_NAME ( FK_NAME ): The foreign key name.
PK_NAME ( UK_NAME ): The primary (unique) key
name.
DEFERRABILITY ( DEFERABILITY ): The deferrability of the for-
eign key constraint. The following codes are defined:
INITIALLY DEFERRED 5
INITIALLY IMMEDIATE 6
NOT DEFERRABLE 7
( UNIQUE_OR_PRIMARY ): This column is necessary if a
driver includes all candidate (i.e. primary and alternate) keys in
the result set (as specified by SQL/CLI). The value of this column
is UNIQUE if the foreign key references an alternate key and PRI-
MARY if the foreign key references a primary key, or it may be
undefined if the driver doesn't have access to the information.
See also "Catalog Methods" and "Standards Reference Information".
"tables"
@names = $dbh->tables( $catalog, $schema, $table, $type );
@names = $dbh->tables; # deprecated
Simple interface to table_info(). Returns a list of matching table
names, possibly including a catalog/schema prefix.
See "table_info" for a description of the parameters.
If "$dbh->get_info(29)" returns true (29 is SQL_IDENTI-
FIER_QUOTE_CHAR) then the table names are constructed and quoted by
"quote_identifier" to ensure they are usable even if they contain
whitespace or reserved words etc. This means that the table names
returned will include quote characters.
"type_info_all"
$type_info_all = $dbh->type_info_all;
Returns a reference to an array which holds information about each
data type variant supported by the database and driver. The array
and its contents should be treated as read-only.
The first item is a reference to an 'index' hash of "Name =">
"Index" pairs. The items following that are references to arrays,
one per supported data type variant. The leading index hash defines
the names and order of the fields within the arrays that follow it.
For example:
$type_info_all = [
{ TYPE_NAME => 0,
DATA_TYPE => 1,
COLUMN_SIZE => 2, # was PRECISION originally
LITERAL_PREFIX => 3,
LITERAL_SUFFIX => 4,
CREATE_PARAMS => 5,
NULLABLE => 6,
CASE_SENSITIVE => 7,
SEARCHABLE => 8,
UNSIGNED_ATTRIBUTE=> 9,
FIXED_PREC_SCALE => 10, # was MONEY originally
AUTO_UNIQUE_VALUE => 11, # was AUTO_INCREMENT originally
LOCAL_TYPE_NAME => 12,
MINIMUM_SCALE => 13,
MAXIMUM_SCALE => 14,
SQL_DATA_TYPE => 15,
SQL_DATETIME_SUB => 16,
NUM_PREC_RADIX => 17,
INTERVAL_PRECISION=> 18,
},
[ 'VARCHAR', SQL_VARCHAR,
undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
],
[ 'INTEGER', SQL_INTEGER,
undef, "", "", undef,0, 0,1,0,0,0,undef,0, 0, 10
],
];
More than one row may have the same value in the "DATA_TYPE" field
if there are different ways to spell the type name and/or there are
variants of the type with different attributes (e.g., with and
without "AUTO_UNIQUE_VALUE" set, with and without
"UNSIGNED_ATTRIBUTE", etc).
The rows are ordered by "DATA_TYPE" first and then by how closely
each type maps to the corresponding ODBC SQL data type, closest
first.
The meaning of the fields is described in the documentation for the
"type_info" method.
An 'index' hash is provided so you don't need to rely on index val-
ues defined above. However, using DBD::ODBC with some old ODBC
drivers may return older names, shown as comments in the example
above. Another issue with the index hash is that the lettercase of
the keys is not defined. It is usually uppercase, as show here, but
drivers may return names with any lettercase.
Drivers are also free to return extra driver-specific columns of
information - though it's recommended that they start at column
index 50 to leave room for expansion of the DBI/ODBC specification.
The type_info_all() method is not normally used directly. The
"type_info" method provides a more usable and useful interface to
the data.
"type_info"
@type_info = $dbh->type_info($data_type);
Returns a list of hash references holding information about one or
more variants of $data_type. The list is ordered by "DATA_TYPE"
first and then by how closely each type maps to the corresponding
ODBC SQL data type, closest first. If called in a scalar context
then only the first (best) element is returned.
If $data_type is undefined or "SQL_ALL_TYPES", then the list will
contain hashes for all data type variants supported by the database
and driver.
If $data_type is an array reference then "type_info" returns the
information for the first type in the array that has any matches.
The keys of the hash follow the same letter case conventions as the
rest of the DBI (see "Naming Conventions and Name Space"). The fol-
lowing uppercase items should always exist, though may be undef:
TYPE_NAME (string)
Data type name for use in CREATE TABLE statements etc.
DATA_TYPE (integer)
SQL data type number.
COLUMN_SIZE (integer)
For numeric types, this is either the total number of digits
(if the NUM_PREC_RADIX value is 10) or the total number of bits
allowed in the column (if NUM_PREC_RADIX is 2).
For string types, this is the maximum size of the string in
characters.
For date and interval types, this is the maximum number of
characters needed to display the value.
LITERAL_PREFIX (string)
Characters used to prefix a literal. A typical prefix is ""'""
for characters, or possibly ""0x"" for binary values passed as
hexadecimal. NULL ("undef") is returned for data types for
which this is not applicable.
LITERAL_SUFFIX (string)
Characters used to suffix a literal. Typically ""'"" for char-
acters. NULL ("undef") is returned for data types where this
is not applicable.
CREATE_PARAMS (string)
Parameter names for data type definition. For example, "CRE-
ATE_PARAMS" for a "DECIMAL" would be ""precision,scale"" if the
DECIMAL type should be declared as "DECIMAL("precision,scale")"
where precision and scale are integer values. For a "VARCHAR"
it would be ""max length"". NULL ("undef") is returned for
data types for which this is not applicable.
NULLABLE (integer)
Indicates whether the data type accepts a NULL value: 0 or an
empty string = no, 1 = yes, 2 = unknown.
CASE_SENSITIVE (boolean)
Indicates whether the data type is case sensitive in collations
and comparisons.
SEARCHABLE (integer)
Indicates how the data type can be used in a WHERE clause, as
follows:
0 - Cannot be used in a WHERE clause
1 - Only with a LIKE predicate
2 - All comparison operators except LIKE
3 - Can be used in a WHERE clause with any comparison operator
UNSIGNED_ATTRIBUTE (boolean)
Indicates whether the data type is unsigned. NULL ("undef") is
returned for data types for which this is not applicable.
FIXED_PREC_SCALE (boolean)
Indicates whether the data type always has the same precision
and scale (such as a money type). NULL ("undef") is returned
for data types for which this is not applicable.
AUTO_UNIQUE_VALUE (boolean)
Indicates whether a column of this data type is automatically
set to a unique value whenever a new row is inserted. NULL
("undef") is returned for data types for which this is not
applicable.
LOCAL_TYPE_NAME (string)
Localized version of the "TYPE_NAME" for use in dialog with
users. NULL ("undef") is returned if a localized name is not
available (in which case "TYPE_NAME" should be used).
MINIMUM_SCALE (integer)
The minimum scale of the data type. If a data type has a fixed
scale, then "MAXIMUM_SCALE" holds the same value. NULL
("undef") is returned for data types for which this is not
applicable.
MAXIMUM_SCALE (integer)
The maximum scale of the data type. If a data type has a fixed
scale, then "MINIMUM_SCALE" holds the same value. NULL
("undef") is returned for data types for which this is not
applicable.
SQL_DATA_TYPE (integer)
This column is the same as the "DATA_TYPE" column, except for
interval and datetime data types. For interval and datetime
data types, the "SQL_DATA_TYPE" field will return "SQL_INTER-
VAL" or "SQL_DATETIME", and the "SQL_DATETIME_SUB" field below
will return the subcode for the specific interval or datetime
data type. If this field is NULL, then the driver does not sup-
port or report on interval or datetime subtypes.
SQL_DATETIME_SUB (integer)
For interval or datetime data types, where the "SQL_DATA_TYPE"
field above is "SQL_INTERVAL" or "SQL_DATETIME", this field
will hold the subcode for the specific interval or datetime
data type. Otherwise it will be NULL ("undef").
Although not mentioned explicitly in the standards, it seems
there is a simple relationship between these values:
DATA_TYPE == (10 * SQL_DATA_TYPE) + SQL_DATETIME_SUB
NUM_PREC_RADIX (integer)
The radix value of the data type. For approximate numeric
types, "NUM_PREC_RADIX" contains the value 2 and "COLUMN_SIZE"
holds the number of bits. For exact numeric types,
"NUM_PREC_RADIX" contains the value 10 and "COLUMN_SIZE" holds
the number of decimal digits. NULL ("undef") is returned either
for data types for which this is not applicable or if the
driver cannot report this information.
INTERVAL_PRECISION (integer)
The interval leading precision for interval types. NULL is
returned either for data types for which this is not applicable
or if the driver cannot report this information.
For example, to find the type name for the fields in a select
statement you can do:
@names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{T
YPE} }
Since DBI and ODBC drivers vary in how they map their types into
the ISO standard types you may need to search for more than one
type. Here's an example looking for a usable type to store a date:
$my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );
Similarly, to more reliably find a type to store small integers,
you could use a list starting with "SQL_SMALLINT", "SQL_INTEGER",
"SQL_DECIMAL", etc.
See also "Standards Reference Information".
"quote"
$sql = $dbh->quote($value);
$sql = $dbh->quote($value, $data_type);
Quote a string literal for use as a literal value in an SQL
statement, by escaping any special characters (such as quotation
marks) contained within the string and adding the required type of
outer quotation marks.
$sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
$dbh->quote("Don't");
For most database types, quote would return 'Dont' (including the
outer quotation marks).
An undefined $value value will be returned as the string "NULL"
(without single quotation marks) to match how NULLs are represented
in SQL.
If $data_type is supplied, it is used to try to determine the
required quoting behaviour by using the information returned by
"type_info". As a special case, the standard numeric types are
optimized to return $value without calling "type_info".
Quote will probably not be able to deal with all possible input
(such as binary data or data containing newlines), and is not
related in any way with escaping or quoting shell meta-characters.
It is valid for the quote() method to return an SQL expression that
evaluates to the desired string. For example:
$quoted = $dbh->quote("one\ntwo\0three")
may return something like:
CONCAT('one', CHAR(12), 'two', CHAR(0), 'three')
The quote() method should not be used with "Placeholders and Bind
Values".
"quote_identifier"
$sql = $dbh->quote_identifier( $name );
$sql = $dbh->quote_identifier( $catalog, $schema, $table, \%attr );
Quote an identifier (table name etc.) for use in an SQL statement,
by escaping any special characters (such as double quotation marks)
it contains and adding the required type of outer quotation marks.
Undefined names are ignored and the remainder are quoted and then
joined together, typically with a dot (".") character. For example:
$id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );
would, for most database types, return "Her schema"."My table"
(including all the double quotation marks).
If three names are supplied then the first is assumed to be a cata-
log name and special rules may be applied based on what "get_info"
returns for SQL_CATALOG_NAME_SEPARATOR (41) and SQL_CATALOG_LOCA-
TION (114). For example, for Oracle:
$id = $dbh->quote_identifier( 'link', 'schema', 'table' );
would return "schema"."table"@"link".
"take_imp_data"
$imp_data = $dbh->take_imp_data;
Leaves the $dbh in an almost dead, zombie-like, state and returns a
binary string of raw implementation data from the driver which
describes the current database connection. Effectively it detaches
the underlying database API connection data from the DBI handle.
After calling take_imp_data(), all other methods except "DESTROY"
will generate a warning and return undef.
Why would you want to do this? You don't, forget I even mentioned
it. Unless, that is, you're implementing something advanced like a
multi-threaded connection pool.
The returned $imp_data can be passed as a "dbi_imp_data" attribute
to a later connect() call, even in a separate thread in the same
process, where the driver can use it to 'adopt' the existing con-
nection that the implementation data was taken from.
Some things to keep in mind...
* the $imp_data holds the only reference to the underlying database
API connection data. That connection is still 'live' and won't be
cleaned up properly unless the $imp_data is used to create a new
$dbh which can then disconnect() normally.
* using the same $imp_data to create more than one other new $dbh
at a time may well lead to unpleasant problems. Don't do that.
The "take_imp_data" method was added in DBI 1.36.
Database Handle Attributes
This section describes attributes specific to database handles.
Changes to these database handle attributes do not affect any other
existing or future database handles.
Attempting to set or get the value of an unknown attribute generates a
warning, except for private driver-specific attributes (which all have
names starting with a lowercase letter).
Example:
$h->{AutoCommit} = ...; # set/write
... = $h->{AutoCommit}; # get/read
"AutoCommit" (boolean)
If true, then database changes cannot be rolled-back (undone). If
false, then database changes automatically occur within a "transac-
tion", which must either be committed or rolled back using the
"commit" or "rollback" methods.
Drivers should always default to "AutoCommit" mode (an unfortunate
choice largely forced on the DBI by ODBC and JDBC conventions.)
Attempting to set "AutoCommit" to an unsupported value is a fatal
error. This is an important feature of the DBI. Applications that
need full transaction behaviour can set "$dbh->{AutoCommit} = 0"
(or set "AutoCommit" to 0 via "connect") without having to check
that the value was assigned successfully.
For the purposes of this description, we can divide databases into
three categories:
Databases which don't support transactions at all.
Databases in which a transaction is always active.
Databases in which a transaction must be explicitly started (C<'BEGI
N WORK'>).
* Databases which don't support transactions at all
For these databases, attempting to turn "AutoCommit" off is a fatal
error. "commit" and "rollback" both issue warnings about being
ineffective while "AutoCommit" is in effect.
* Databases in which a transaction is always active
These are typically mainstream commercial relational databases with
"ANSI standard" transaction behaviour. If "AutoCommit" is off,
then changes to the database won't have any lasting effect unless
"commit" is called (but see also "disconnect"). If "rollback" is
called then any changes since the last commit are undone.
If "AutoCommit" is on, then the effect is the same as if the DBI
called "commit" automatically after every successful database oper-
ation. So calling "commit" or "rollback" explicitly while "AutoCom-
mit" is on would be ineffective because the changes would have
already been commited.
Changing "AutoCommit" from off to on will trigger a "commit".
For databases which don't support a specific auto-commit mode, the
driver has to commit each statement automatically using an explicit
"COMMIT" after it completes successfully (and roll it back using an
explicit "ROLLBACK" if it fails). The error information reported
to the application will correspond to the statement which was exe-
cuted, unless it succeeded and the commit or rollback failed.
* Databases in which a transaction must be explicitly started
For these databases, the intention is to have them act like
databases in which a transaction is always active (as described
above).
To do this, the driver will automatically begin an explicit trans-
action when "AutoCommit" is turned off, or after a "commit" or
"rollback" (or when the application issues the next database opera-
tion after one of those events).
In this way, the application does not have to treat these databases
as a special case.
See "commit", "disconnect" and "Transactions" for other important
notes about transactions.
"Driver" (handle)
Holds the handle of the parent driver. The only recommended use for
this is to find the name of the driver using:
$dbh->{Driver}->{Name}
"Name" (string)
Holds the "name" of the database. Usually (and recommended to be)
the same as the ""dbi:DriverName:..."" string used to connect to
the database, but with the leading ""dbi:DriverName:"" removed.
"Statement" (string, read-only)
Returns the statement string passed to the most recent "prepare"
method called in this database handle, even if that method failed.
This is especially useful where "RaiseError" is enabled and the
exception handler checks $@ and sees that a 'prepare' method call
failed.
"RowCacheSize" (integer)
A hint to the driver indicating the size of the local row cache
that the application would like the driver to use for future
"SELECT" statements. If a row cache is not implemented, then set-
ting "RowCacheSize" is ignored and getting the value returns
"undef".
Some "RowCacheSize" values have special meaning, as follows:
0 - Automatically determine a reasonable cache size for each C<SELEC
T>
1 - Disable the local row cache
>1 - Cache this many rows
<0 - Cache as many rows that will fit into this much memory for each
C<SELECT>.
Note that large cache sizes may require a very large amount of mem-
ory (cached rows * maximum size of row). Also, a large cache will
cause a longer delay not only for the first fetch, but also when-
ever the cache needs refilling.
See also the "RowsInCache" statement handle attribute.
"Username" (string)
Returns the username used to connect to the database.
DBI STATEMENT HANDLE OBJECTS
This section lists the methods and attributes associated with DBI
statement handles.
Statement Handle Methods
The DBI defines the following methods for use on DBI statement handles:
"bind_param"
$sth->bind_param($p_num, $bind_value)
$sth->bind_param($p_num, $bind_value, \%attr)
$sth->bind_param($p_num, $bind_value, $bind_type)
The "bind_param" method takes a copy of $bind_value and associates
it (binds it) with a placeholder, identified by $p_num, embedded in
the prepared statement. Placeholders are indicated with question
mark character ("?"). For example:
$dbh->{RaiseError} = 1; # save having to check each method ca
ll
$sth = $dbh->prepare("SELECT name, age FROM people WHERE name LIKE ?
");
$sth->bind_param(1, "John%"); # placeholders are numbered from 1
$sth->execute;
DBI::dump_results($sth);
See "Placeholders and Bind Values" for more information.
Data Types for Placeholders
The "\%attr" parameter can be used to hint at the data type the
placeholder should have. Typically, the driver is only interested
in knowing if the placeholder should be bound as a number or a
string.
$sth->bind_param(1, $value, { TYPE => SQL_INTEGER });
As a short-cut for the common case, the data type can be passed
directly, in place of the "\%attr" hash reference. This example is
equivalent to the one above:
$sth->bind_param(1, $value, SQL_INTEGER);
The "TYPE" value indicates the standard (non-driver-specific) type
for this parameter. To specify the driver-specific type, the driver
may support a driver-specific attribute, such as "{ ora_type => 97
}".
The SQL_INTEGER and other related constants can be imported using
use DBI qw(:sql_types);
See "DBI Constants" for more information.
The data type for a placeholder cannot be changed after the first
"bind_param" call. In fact the whole \%attr parameter is 'sticky'
in the sense that a driver only needs to consider the \%attr param-
eter for the first call, for a given $sth and parameter. After that
the driver may ignore the \%attr parameter for that placeholder.
Perl only has string and number scalar data types. All database
types that aren't numbers are bound as strings and must be in a
format the database will understand except where the bind_param()
TYPE attribute specifies a type that implies a particular format.
For example, given:
$sth->bind_param(1, $value, SQL_DATETIME);
the driver should expect $value to be in the ODBC standard
SQL_DATETIME format, which is 'YYYY-MM-DD HH:MM:SS'. Similarly for
SQL_DATE, SQL_TIME etc.
As an alternative to specifying the data type in the "bind_param"
call, you can let the driver pass the value as the default type
("VARCHAR"). You can then use an SQL function to convert the type
within the statement. For example:
INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))
The "CONVERT" function used here is just an example. The actual
function and syntax will vary between different databases and is
non-portable.
See also "Placeholders and Bind Values" for more information.
"bind_param_inout"
$rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len) or die
$sth->errstr;
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr)
or ...
$rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_t
ype) or ...
This method acts like "bind_param", but also enables values to be
updated by the statement. The statement is typically a call to a
stored procedure. The $bind_value must be passed as a reference to
the actual value to be used.
Note that unlike "bind_param", the $bind_value variable is not
copied when "bind_param_inout" is called. Instead, the value in the
variable is read at the time "execute" is called.
The additional $max_len parameter specifies the minimum amount of
memory to allocate to $bind_value for the new value. If the value
returned from the database is too big to fit, then the execution
should fail. If unsure what value to use, pick a generous length,
i.e., a length larger than the longest value that would ever be
returned. The only cost of using a larger value than needed is
wasted memory.
Undefined values or "undef" are used to indicate null values. See
also "Placeholders and Bind Values" for more information.
"bind_param_array"
$rc = $sth->bind_param_array($p_num, $array_ref_or_value)
$rc = $sth->bind_param_array($p_num, $array_ref_or_value, \%attr)
$rc = $sth->bind_param_array($p_num, $array_ref_or_value, $bind_type
)
The "bind_param_array" method is used to bind an array of values to
a placeholder embedded in the prepared statement which is to be
executed with "execute_array". For example:
$dbh->{RaiseError} = 1; # save having to check each method ca
ll
$sth = $dbh->prepare("INSERT INTO staff (first_name, last_name, dept
) VALUES(?, ?, ?)");
$sth->bind_param_array(1, [ 'John', 'Mary', 'Tim' ]);
$sth->bind_param_array(2, [ 'Booth', 'Todd', 'Robinson' ]);
$sth->bind_param_array(3, "SALES"); # scalar will be reused for each
row
$sth->execute_array( { ArrayTupleStatus => \my @tuple_status } );
The %attr ($bind_type) argument is the same as defined for
"bind_param". Refer to "bind_param" for general details on using
placeholders.
(Note that bind_param_array() can not be used to expand a place-
holder into a list of values for a statement like "SELECT foo WHERE
bar IN (?)". A placeholder can only ever represent one value per
execution.)
Scalar values, including "undef", may also be bound by
"bind_param_array". In which case the same value will be used for
each "execute" call. Driver-specific implementations may behave
differently, e.g., when binding to a stored procedure call, some
databases may permit mixing scalars and arrays as arguments.
The default implementation provided by DBI (for drivers that have
not implemented array binding) is to iteratively call "execute" for
each parameter tuple provided in the bound arrays. Drivers may
provide more optimized implementations using whatever bulk opera-
tion support the database API provides. The default driver
behaviour should match the default DBI behaviour, but always con-
sult your driver documentation as there may be driver specific
issues to consider.
Note that the default implementation currently only supports non-
data returning statements (INSERT, UPDATE, but not SELECT). Also,
"bind_param_array" and "bind_param" cannot be mixed in the same
statement execution, and "bind_param_array" must be used with "exe-
cute_array"; using "bind_param_array" will have no effect for "exe-
cute".
The "bind_param_array" method was added in DBI 1.22.
"execute"
$rv = $sth->execute or die $sth->errstr;
$rv = $sth->execute(@bind_values) or die $sth->errstr;
Perform whatever processing is necessary to execute the prepared
statement. An "undef" is returned if an error occurs. A success-
ful "execute" always returns true regardless of the number of rows
affected, even if it's zero (see below). It is always important to
check the return status of "execute" (and most other DBI methods)
for errors if you're not using "RaiseError".
For a non-"SELECT" statement, "execute" returns the number of rows
affected, if known. If no rows were affected, then "execute"
returns "0E0", which Perl will treat as 0 but will regard as true.
Note that it is not an error for no rows to be affected by a state-
ment. If the number of rows affected is not known, then "execute"
returns -1.
For "SELECT" statements, execute simply "starts" the query within
the database engine. Use one of the fetch methods to retrieve the
data after calling "execute". The "execute" method does not return
the number of rows that will be returned by the query (because most
databases can't tell in advance), it simply returns a true value.
If any arguments are given, then "execute" will effectively call
"bind_param" for each value before executing the statement. Values
bound in this way are usually treated as "SQL_VARCHAR" types unless
the driver can determine the correct type (which is rare), or
unless "bind_param" (or "bind_param_inout") has already been used
Edit Notes
- 2006-06-09 Transcribed up to Manual page DBI(3pm) line 3093