Database trigger

A database trigger is procedural code that is automatically executed in response to certain events on a particular table or view in a database. The trigger is mostly used for maintaining the integrity of the information on the database. For example, when a new record (representing a new worker) is added to the employees table, new records should also be created in the tables of the taxes, vacations and salaries.

Below follows a series of descriptions of how some popular DBMS support triggers.

In addition to triggers that fire when data is modified, Oracle 10g supports triggers that fire when schema level objects (that is, tables) are modified and when user logon or logoff events occur. These trigger types are referred to as "Schema-level triggers".

The four main types of triggers are:

A full list is available on MSDN.

Performing conditional actions in triggers (or testing data following modification) is done through accessing the temporary Inserted and Deleted tables.

Introduced support for triggers in 1997. The following functionality in SQL:2003 was previously not implemented in PostgreSQL:

Synopsis:

Firebird supports multiple row-level, BEFORE or AFTER, INSERT, UPDATE, DELETE (or any combination thereof) triggers per table, where they are always "in addition to" the default table changes, and the order of the triggers relative to each other can be specified where it would otherwise be ambiguous (POSITION clause.) Triggers may also exist on views, where they are always "instead of" triggers, replacing the default updatable view logic. (Before version 2.1, triggers on views deemed updatable would run in addition to the default logic.)

Firebird does not raise mutating table exceptions (like Oracle), and triggers will by default both nest and recurse as required (SQL Server allows nesting but not recursion, by default.) Firebird's triggers use NEW and OLD context variables (not Inserted and Deleted tables,) and provide UPDATING, INSERTING, and DELETING flags to indicate the current usage of the trigger.

...
Wikipedia