19.3. Using Triggers

A trigger is a named database object that is associated with a table, and that activates when a particular event occurs for the table. Some uses for triggers are to perform checks of values to be inserted into a table or to perform calculations on values involved in an update.

A trigger is defined to activate when an INSERT, DELETE, or UPDATE statement executes for the associated table. A trigger can be set to activate either before or after the triggering statement. For example, you can have a trigger activate before each row that is inserted into a table or after each row that is updated.

Important

MySQL triggers are activated by SQL statements only. They are not activated by changes in views, nor by changes to tables made by APIs that do not transmit SQL statements to the MySQL Server. This means that triggers are not activated by changes in INFORMATION_SCHEMA or performance_schema tables, because these tables are actually views.

To use triggers if you have upgraded to MySQL 5.5 from an older release that did not support triggers, you should upgrade your grant tables so that they contain the trigger-related privileges. See Section 4.4.7, “mysql_upgrade — Check Tables for MySQL Upgrade”.

The following discussion describes the syntax for creating and dropping triggers, and shows some examples of how to use them.

Additional Resources

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