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