(DML-Data Manipulation Language statements are used for

managing data within schema objects…

-{select,insert,update,delete,merge,call,explain,lock table} )…

Syntax of trigger:—-

CREATE OR REPLACE TRIGGER trigger_name

AFTER DELETE OR INSERT OR UPDATE OF column_name

ON table_name

FOR EACH ROW BEGIN
sql statement END;

Types of triggers:-
1-Row trigger and Statement triggers
2-Before and After triggers
3-Instead of triggers
4-Triggers on System events and User events.

Example:–
The total_salary trigger maintains a derived column that
stores the total salary of all members in a department:

CREATE TRIGGER total_salary

AFTER DELETE OR INSERT OR UPDATE OF department_id, salary ON employees

FOR EACH ROW BEGIN

IF DELETING OR (UPDATING AND old.department_id != new.department_id)

THEN UPDATE departments

SET total_salary = total_salary – old.salary

WHERE department_id = old.department_id;

END IF;

IF INSERTING OR (UPDATING AND old.department_id != new.department_id)

THEN UPDATE departments

SET total_salary = total_salary + new.salary

WHERE department_id = new.department_id;

END IF;

IF (UPDATING AND old.department_id = new.department_id AND

old.salary != new.salary )

THEN UPDATE departments

SET total_salary = total_salary – old.salary + new.salary

WHERE department_id = new.department_id;

END IF;