Answer Posted / ranganath
In a DBMS, a trigger is a SQL procedure that initiates an
action (i.e., fires an action) when an event (INSERT,
DELETE or UPDATE) occurs. Since triggers are event-driven
specialized procedures, they are stored in and managed by
the DBMS. A trigger cannot be called or executed; the DBMS
automatically fires the trigger as a result of a data
modification to the associated table. Triggers are used to
maintain the referential integrity of data by changing the
data in a systematic fashion.
Each trigger is attached to a single, specified table in
the database.
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.
A trigger can also contain INSERT, UPDATE and DELETE logic
within itself, so when the trigger is fired because of data
modification it can also cause another data modification,
thereby firing another trigger. A trigger that contains
data modification logic within itself is called a nested
trigger.
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