A trigger is a named database object that is associated with a table and that is activated when a particular event occurs for the table. For example, the following statements create a table and an INSERT trigger. The trigger sums the values inserted into one of the table's columns:

mysql> CREATE TABLE account (acct_num INT, amount DECIMAL(10,2));
Query OK, 0 rows affected (0.03 sec)

mysql> CREATE TRIGGER ins_sum BEFORE INSERT ON account
    -> FOR EACH ROW SET @sum = @sum + NEW.amount;
Query OK, 0 rows affected (0.06 sec)

This chapter describes the syntax for creating and dropping triggers, and shows some examples of how to use them. Discussion of restrictions on use of triggers is given in Section D.1, “Restrictions on Stored Routines, Triggers, and Events”. Remarks regarding binary logging as it applies to triggers are given in Section 22.4, “Binary Logging of Stored Routines and Triggers”.

For answers to some common questions about triggers in MySQL 5.1, see Section A.5, “MySQL 5.1 FAQ — Triggers”.