MySQL/notes

Repairing

• mysqlcheck -A --auto-repair -p

SQL error messages

• 1271 - Illegal mix of collations for operation 'concat'
  • http://www.issociate.de/board/post/274187/CONCAT_with_IF.html
  • When the problem expression was a CONCAT(...) function, doing CAST(CONCAT(...) AS BINARY) fixed it.

munged MySQL installation

If you can't reinstall or remove MySQL because the "/etc/init.d/mysql start" command fails during (un)installation, check for rogue "mysql" and "mysql-safe" processes which aren't getting stopped by "/etc/init.d/mysql stop".

debian-start

The /etc/mysql/debian-start file appears to contain a minor error, at least on the MySQL install I just reinstalled. It sets the variable MYCHECK, and then tries to invoke the command to which it is set by saying just "mycheck;", resulting in the error message "/etc/mysql/debian-start: line 17: mycheck: command not found" although the mysql daemon is apparently restarted successfully. To fix the error, change "mycheck;" to "$mycheck;". (This also has the benefit of actually running whatever command MYCHECK is set to, which is typically some kind of integrity check to help keep your databases from becoming corrupted.)

external connections

If mysql refuses to respond to connection attempts from anywhere but the local machine, you need to edit /etc/mysql/my.conf and comment out the following line:

bind-address  = 127.0.0.1

and then restart mysqld.

miscellaneous

To restart the mysql daemon (mysqld) when you don't have a user with SHUTDOWN privileges but you do have system root:

sudo /etc/init.d/mysql restart

Command-line Client Example

editing a blank table: To work around the Query Browser bug (fixed in the latest version) which won't let you edit a blank table, create the first entry using MySQL's command line interface:

woozle@camilla:~ $ mysql -hyourhost -uyourusername -pyourpassword
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 12531702 to server version: 4.1.13-standard

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql> use yourdatabasename
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> INSERT INTO tablename () VALUES();
Query OK, 1 row affected (0.05 sec)

mysql>

...where yourhost is the name of your MySQL server, and can be anything that can resolve to an IP address, e.g. localhost, yourmachinename, yourserverdomain.com, or an actual IP address (I think).

SQL Examples

creating a table: If the Query Browser crashes when you try to create or edit a table, use the command line interface to create the table:

create table table_name (field_name field_type, field_name field_type[ <, ...> ]);

Example: CREATE TABLE example (key varchar(255));

• More examples: importing data into ZenCart
• Documentation: CREATE TABLE Syntax

making a field autonumbered:

ALTER TABLE tablename MODIFY fieldname fieldtype AUTO_INCREMENT;

Example: ALTER TABLE Tracks_Queued MODIFY id INTEGER AUTO_INCREMENT;

changing the name of a field (column):

ALTER TABLE tablename CHANGE oldname newname new_fieldtype;

This also changes the field's definition, if new_fieldtype is not the same as the field's prior definition.

Example: ALTER TABLE cat_items CHANGE isInStock qtyInStock INT COMMENT 'number of pieces currently in stock (calculated from stock table)';

• Documentation: ALTER TABLE Syntax

Address Binding

If you get this message on attempting to connect to a remote mysql server (named "rizzo" in this case):

ERROR 2003 (HY000): Can't connect to MySQL server on 'rizzo' (111)

...and "telnet rizzo 3306" returns this (where rizzo is at 192.168.0.103):

Trying 192.168.0.103...
telnet: Unable to connect to remote host: Connection refused

...then the problem may be that mysql is only "listening" on its loopback interface. Look in the my.cnf file for:

bind-address		= 127.0.0.1

...and change it to:

bind-address	= 192.168.0.103

...using your mysql server's actual IP address instead of the one given.

It's not clear if there's any address-nonspecific way of doing this so that (e.g.) if rizzo were to be assigned a different IP address later on, my.cnf would not need to be changed.

MySQL on a DNS server

Some observations on 2007-09-21, from the owner/operator of a small web hosting provider. These only apply if:

• the web sites are on a different machine from the MySQL server
• the web site machine is connected to the MySQL server directly (typically via crossover cable), rather than going through the LAN

What happened, and the solution (in brief):

Tonight, my cluster bit the dust. Every time we would restart MySQL, we would immediately be deluged with huge amounts of "unauthenticated users" hitting up the DB server. Thanks to the good folks in #MySQL on EFNet IRC, they turned me onto http://hackmysql.com/dns

For /etc/my.cnf

After [mysqld]

ADD this, on its own line: skip-name-resolve

---------------------

If you're running your DNS on the same machine as your DB server, make SURE you add the three following lines to /etc/resolv.conf:

nameserver 127.0.0.1

nameserver 192.168.0.10

nameserver 192.168.0.20

Modifying the two 192.168.x.x ranges to be the IP's of your internal private network.

At one point, my DB server was deluged with over 2300 requests that were trying to do reverse DNS on 192.168.0.10 (which is the web server, .20 is the DB server). Since the entire 192.168.X.X block is set aside for internal private networks, chances are that you will NOT have those resolving into hostnames. The problem was that my DB server was trying to resolve all 2300 requests into a hostname before it allowed DB queries to go through.

You'll have to restart (or shut down/kill off all MySQL processes, remove mysql.sock and your pid file) and then start MySQL up again.

I hope this saves someone the two hours of headache I've just gone through getting this sorted.

Full post is here.