create or replace trigger salesman_com_only
  before insert or update on empy
  for each row
begin
  if :new.job = 'SALESMAN' then
    if inserting or (updating and :old.job <> 'SALESMAN') then
      :new.comm := 0;
    end if;
  else    
    if :new.comm is not null then
      raise_application_error(-20501, 'Only salesman can have a commission');
    end if;  
  end if;
end;
/