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; the 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.
Nested Trigger: 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.