Triggers are is a named PL/SQL unit that is stored in the database and can be invoked repeatedly.While a trigger is enabled, the database automatically invokes it—that is, the trigger fires—whenever its triggering event occurs. While a trigger is disabled, it does not fire.

Follwing is the simple example of trigger, whenever any oprations like insert,update or delete is performed on the table emp, the AIUD_EMP will be fired automatically and will print the appropriate action message.

create or replace trigger AIUD_EMP 
after insert or update or delete on EMP 
for each row 
begin 
    if inserting then 
        DBMS_OUTPUT.PUT_LINE('Record is Added'); 
    end if; 
    if updating then 
        DBMS_OUTPUT.PUT_LINE('Record is Updated'); 
    end if; 
    if deleting then 
        DBMS_OUTPUT.PUT_LINE('Record is Deleted'); 
    end if; 
end;