Usage of the when statement in Oracle triggers

Case: The salesperson's salary in the Scott. EMP table can only increase, but cannot decrease

Code:

Create or replace trigger tr_emp_upd
Before
Update of Sal
On EMP
For each row

-- Triggered only when the salesperson is updated
When
(Old. Job = 'salesman ')
Declare
Begin
If (: New. Sal <: Old. Sal)
Then
Raise_application_error (-20013, 'the Salesperson's salary cannot be lowered! ');
End
If;
End;

 

Running status:
SQL> Update EMP set
Sal = Sal-1000 where job = 'manager ';

3 rows updated

SQL> Update EMP set sal = Sal-1000 where
Job = 'salesman ';

ORA-20013: salesman wages can not be reduced!
ORA-06512:
In "Scott. tr_emp_upd", line 4
ORA-04088: An error occurred while executing the trigger 'Scott. tr_emp_upd'