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 a statement inserts, updates,
or deletes rows in the associated table. These row operations are
trigger events. For example, rows can be inserted by
DATA statements, and an insert trigger activates for each
inserted row. A trigger can be set to activate either before or
after the trigger event. For example, you can have a trigger
activate before each row that is inserted into a table or after each
row that is updated.
MySQL triggers activate only for changes made to tables by SQL statements. They do not activate for 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 tables, because these
tables are actually views.
Triggers are not activated by updates made using the
To use triggers if you have upgraded to MySQL 5.1 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 4.4.8, “mysql_upgrade — Check and Upgrade MySQL Tables”.
The following sections describe the syntax for creating and dropping triggers, show some examples of how to use them, and indicate how to obtain trigger metadata.
You may find the Triggers User Forum of use when working with triggers.
For answers to commonly asked questions regarding triggers in MySQL, see Section A.5, “MySQL 5.1 FAQ: Triggers”.
There are some restrictions on the use of triggers; see Section D.1, “Restrictions on Stored Programs”.
Binary logging for triggers takes place as described in Section 19.7, “Binary Logging of Stored Programs”.