Instead of Trigger

Scott wants to create a view of salary totals by department. He will need to be granted that privilege by System.

SQL> Connect System/******
Connected.
SQL> Grant Create View To Scott;
Grant succeeded.
SQL> Connect Scott/Tiger
Connected.

Create or Replace View v_TotSal
    As
       Select DeptNo
         ------------------
       ,   Sum(Sal) As
         SumSal
         ------------------
        From Emp
        Group By DeptNo
        Order By DeptNo;

View created.

SQL> Select *
  2    From v_TotSal;

DEPTNO    SUMSAL
------ ---------
    10    $9,268
    20   $11,065
    30   $10,055

If you try to delete one of the rows in this view, you are stopped with an ORA error.

SQL> Delete From v_TotSal
  2   Where DeptNo = 10;
Delete From v_TotSal
            *
ERROR at line 1:
ORA-01732: data manipulation operation not legal on 
                           this view

Since you can't delete directly from that view, you'll need to create a trigger that will delete from the base table based on the input given.

Create Or Replace Trigger v_TotSal_Del
  Instead Of Delete On v_TotSal
  For Each Row
Begin
 Delete From Emp
  Where DeptNo = :Old.DeptNo;
End;
/

Trigger created.

SQL> Delete From v_TotSal
  2   Where DeptNo = 10;

1 row deleted.

This time, it was successful.

SQL> Select Distinct DeptNo
  2    From Emp;

DEPTNO
------
    30
    20

Trigger: Check Values Before Data Entry

This trigger is fired whenever the JOB column is inserted or updated on the EMP table.

Create or Replace Trigger t_Top_Dog
  Before Insert or Update
    Of Job
      On Emp
        For Each Row
Begin
    If :New.Job = 'PRESIDENT'
      Then Raise_Application_Error 
                (-20111,'Only one top dog allowed.');
    End If;
End;
/

Trigger created.

SQL> Insert Into Emp Values
  2  (7998,'TOBIAS','DEVELOPER',NULL,SYSDATE,3500,NULL,10);

1 row created.

SQL> Insert Into Emp Values
  2  (7999,'ELMO','PRESIDENT',NULL,SYSDATE,6000,NULL,20);
Insert Into Emp Values
            *
ERROR at line 1:
ORA-20111: Only one top dog allowed.
ORA-06512: at "SCOTT.T_TOP_DOG", line 3
ORA-04088: error during execution of trigger 'SCOTT.T_TOP_DOG'