A trigger is a procedure body associated with a table and an event. A trigger can take effect before, after or instead of the event on the subject table. Several before, after or instead of triggers may exist for a given event on a given table, which can be fired in a specified order.

Triggers are useful for enforcing integrity rules, maintaining the validity of data computed from other data, accumulating history data etc.

A trigger body has no arguments in the sense a procedure does. A trigger body implicitly sees the columns of the subject table as read-only parameters. An update trigger may see both the new and old values of the row of the subject table. These are differentiated by correlation names in the REFERENCING clause.

Triggers are capable of cascading; the code of a trigger may cause another trigger to be activated. This may lead to non-terminating recursion in some cases. Triggers may be turned off either inside a compound statement or inside a connection with the SET TRIGGERS OFF statement.

An update trigger may have a set of sensitive columns whose update will cause the trigger code to be run. Update of non-sensitive columns will not invoke the trigger. If no column list is specified any update will invoke the trigger.