Examples

SQL Tutorial, Database Triggers

Explanation

Triggers are special database procedures that run implicitly after a data manipulation statement has been executed on a base table (or a view table based on this base table). A system trigger is a special database procedure performed implicitly during a restart. The conditions under which a trigger is to be executed can be restricted further. A trigger can call further triggers implicitly.

The trigger is executed for each row to which the SQL statement refers. The trigger can access both the old values (values before update or deletion) and the new values (values after update or insertion) in this row.

Triggers can be used to check complicated integrity rules, initiate derived database modifications for the row in question, or implement complex access protection rules.

The database system provides a language (special SQL syntax that has been extended to include variables, control structures, and troubleshooting measures) that allows an SQL access layer to be formulated on the server side. This special SQL syntax can be used to define triggers.