re: trigger

hi guys how can i write a trigger on myemp tabl to;

1. Sales persons should always receive commission. Employees who are not sales persons should never receive a commission;

2. Salaries may only be increased, never decreased.

I just type a trigger for:
(2)Salaries may only be increased, never decreased.

CRAETE TRIGGER salary_check
BEFORE
INSERT OR UPDATE OF salary ON myemp
FOR EACH ROW
DECLARE
currsal number;
BEGIN
SELECT salary INTO currsal
FROM myemp
WHERE employee# = :new.employee#;

IF (:new.sal < currsal) THEN
raise_application_error (-20888, 'New salary for this empoyee ' || :new.employee# || ' must be greater than its current salary');
END IF;
END;
/

Regards