What is Trigger?

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A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or
UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to
maintain the referential integrity of data by changing the data in a systematic fashion. A
trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a
data modification to the associated table. Triggers can be viewed as similar to stored
procedures in that both consist of procedural logic that is stored at the database level.
Stored procedures, however, are not event-drive and are not attached to a specific table as
triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure
while triggers are implicitly executed. In addition, triggers can also execute stored
procedures.