MySQL’s, and MariaDB’s, strict mode controls how invalid or missing values in data changing queries are handled; this includes INSERT, UPDATE, and CREATE TABLE statements. With MySQL strict mode enabled, which is the default state, invalid or missing data may cause warnings or errors when attempting to process the query.

MySQL has "modes", each of which enable or disable a certain behavior. For example, ERROR_FOR_DIVISION_BY_ZERO is a mode that, you guessed it, throws an error when you divide by zero in a SQL division operation. Without this mode enabled, you'll just get a NULL result silently.

"Strict mode" which is really just the list of modes in MySQL 5.7 enables by default, is comprised of the following modes:

ONLY_FULL_GROUP_BY
STRICT_TRANS_TABLES
NO_ZERO_IN_DATE
NO_ZERO_DATE
ERROR_FOR_DIVISION_BY_ZERO
NO_AUTO_CREATE_USER
NO_ENGINE_SUBSTITUTION

When strict mode is disabled the same query would have its invalid, or missing, values adjusted and would produce a simple warning. This may seem like the preferred result, however with strict mode disabled certain actions may cause unexpected results; for instance, when the value being inserted exceeds the maximum character limit it will be truncated to fit the limit.

There are various reasons why MySQL’s strict mode may need to be disabled, however the most common is when a server is running WHMCS (https://www.whmcs.com/) — this is a requirement of that tool.

Pre-Flight Check

• These instructions are intended specifically for disabling MySQL strict mode on a managed Linux server with cPanel.
• The server should be running either MySQL 5.6/5.7 or MariaDB 10.x.
• Command line and root level access via SSH will be necessary to follow this tutorial.

Step #1: Make Backups, Always!

Whenever modifying files on a server it’s always best practice to take some form of a backup beforehand. This ensures you have a way to revert changes if something goes awry; it’s also beneficial because it helps track when and what changes were made.

While logged into SSH with the root user, do the following:

cp -a /usr/my.cnf{,.strict.bak}
cp -a /etc/my.cnf{,.strict.bak}

The above command uses BASH brace expansion in order to make a backup copy of the file in its original directory. Brace expansion is used to generate arbitrary strings. The specified strings are used to generate all possible combinations with the optional surrounding preambles and postscripts.

Step #2: Disable MySQL Strict Mode

Depending on the server and the current configurations you may need to edit one, or both, of the following files on the server. Generally, the relevant configuration lines are only in one of them, however, it could be in either one without causing issues; so generally it’s best to check both.

To edit the files, you will open the file with your favorite command line editor. In this example, we use ‘vim’.

vim /usr/my.cnf
vim /etc/my.cnf

In vim, you can press “a” or “i” to enter text insertion mode; pressing the escape key (Esc) on your keyboard returns you to command mode. Within each file above you will be looking for a line with the following content:

sql_mode=NO_ENGINE_SUBSTITUTION,STRICT_TRANS_TABLES

If you find a line similar to the above that is setting the `sql_mode` variable then you will need to replace it with the following line to disable MySQL strict mode.

sql_mode=""

Once this adjustment has been made, or you’ve confirmed the file does not need to be adjusted you will then save and close the file.

Step #3: Restart the MySQL Service

Finally, to make these changes effective you will need to restart the MySQL service as it will only read the configuration files when it initially loads up. In order to force MySQL to use the new configuration files you will do the following:

For CentOS 7 servers:

systemctl restart mysql

For CentOS 6 and prior:

/etc/init.d/mysql restart

After issuing this command on the server the MySQL service will be restarted and will load the changes made. If all the directions were followed and completed, then MySQL strict mode should now be disabled.

To verify that the process was completed properly you can run the following:

mysql -e "SELECT @@sql_mode;"

The output may look similar to the following:

+--------------------------------------------+
| @@sql_mode
+--------------------------------------------+
| NO_AUTO_CREATE_USER
+--------------------------------------------+