DECLARE
   v_cant_e  NUMBER (5);
   v_cant_s  NUMBER (5);
   v_total   NUMBER (5);
BEGIN
   BEGIN
      SELECT SUM (quantity)
        INTO v_cant_e
        FROM mov
       WHERE     cod_mov = :mov.cod_mov
             AND date_mov <= :mov.date_mov
             AND signo = '+';
   EXCEPTION
      WHEN OTHERS
      THEN
         v_cant_e := 0;
   END;

   BEGIN
      SELECT SUM (quantity)
        INTO v_cant_s
        FROM mov
       WHERE     cod_mov = :mov.cod_mov
             AND date_mov <= :mov.date_mov
             AND signo = '-';
   EXCEPTION
      WHEN OTHERS
      THEN
         v_cant_s := 0;
   END;

   v_total := v_cant_e - v_cant_s;

   IF v_total < 0
   THEN
      MESSAGE ('quantity exceeded on Article ' || :mov.cod_mov);
      MESSAGE ('quantity exceeded on Article ' || :mov.cod_mov);

      RAISE form_trigger_failure;
   END IF;
END;