What is Trigger?

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.

 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.