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.