Paul Turner » MySQL

MySQL compare NULL equivalent to an empty string

Paul — Mon, 27 Sep 2010 13:08:10 +0000

Nearly all MySQL’s string functions return NULL when a NULL value is used for comparison. Although this is correct sometimes it is useful to compare NULL as being equalivalent to an empty string (although they are technically different).

One exception in MySQL’s string functions is the CONACT_WS function as this skips NULL parameters after the separator argument.

This means it can be used like this to compare NULL as being equal to an empty string:

CONCAT_WS('',value1) = CONCAT_WS('',value2)

This will return true if value1 and value2 are any combination of NULL and empty string.

MySQL slow query log

Paul — Mon, 13 Oct 2008 16:34:34 +0000

The MySQL slow query log enables you to log all queries that take longer than a specified number of seconds to execute. This should help identify any queries that are not performing sufficiently.

All that is required is two additional lines in the MySQL configuration file “my.cnf” in the [mysqld] section.

[mysqld]
log-slow-queries = /var/log/mysql-slow.log
long_query_time = 4

The above configuration will log any queries taking longer than 4 seconds to execute in to the specified log file.

MySQL appears to require that the specified log file exists otherwise the logging is disabled and the error below appears in the MySQL log. (Or it may be that it didn’t have permissions to create the file. I haven’t investigated).

081013 14:55:37 [ERROR] Could not use /var/log/mysql-slow.log for logging (error 13). Turning logging off for the whole duration of the MySQL server process. To turn it on again: fix the cause, shutdown the MySQL server and restart it.

To create the log file I did the following:

touch /var/log/mysql-slow.log
chown mysql.mysql /var/log/mysql-slow.log
chmod o-r /var/log/mysql-slow.log

The third command is probably unnecessary, but it meant that the permissions matched the mysqld.log in the same directory.

Once this is done MySQL needs to be restarted to start the query logging:

service mysql restart

MySQL export to CSV

Paul — Mon, 02 Jun 2008 15:12:55 +0000

MySQL allows you to export a query directly in to a CSV file that can be opened by Microsoft Excel.

This is the basic syntax:

SELECT FIELDS FROM TABLE INTO OUTFILE '/path/to/file_name.csv' FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' LINES TERMINATED BY '\n';

The export file that is created “file_name.csv” must not already exist, the MySQL user must have write permissions to the directory the file will be created in, and the FILE permission within MySQL.

See the MySQL manual: SELECT Syntax

Creating a MySQL unicode UTF-8 database

Paul — Sat, 03 May 2008 11:44:14 +0000

Here’s the syntax for creating a UTF-8 database in MySQL as I can never remember it!

CREATE DATABASE `databasename` /*!40100 DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci */;