In Chapter 5, I described three types of stored subprograms: procedures, functions, and packages. In addition to these subprogram types, PL/SQL enables a fourth type of stored subprogram called a database trigger. A database trigger is a stored subprogram that’s stored in the database and executed implicitly on the occurrence of an event. The event can be a DDL operation, such as an object creation, alteration, or dropping; a DML operation, such an INSERT, an UPDATE, or a DELETE on a table or view; a system event, such as database startup and shutdown, logon, and logoff; or a user event, such as schema logon and logoff.
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