TRIGGER

CREATE TABLE RESERVA_PRUEBA
(
COD_RESERVA  NUMBER PRIMARY KEY,
DESCRIPCION  VARCHAR2(50) NOT NULL,
FECHA       TIMESTAMP NOT NULL,
MONTO        INT NULL,
HORAS INT NULL,
COD_CLIENTE         NUMBER REFERENCES CLIENTE,
COD_EMP                  VARCHAR2(30) REFERENCES EMPLEADO,
COD_CANALCOM    NUMBER REFERENCES CANAL_COMUNICACION,
SUB_TOTAL INT
);

INSERT INTO RESERVA_PRUEBA VALUES(1, 'Reserva Familiar: 7 personas', '15-feb-18 11:01:11',20, 5, 0001, 'EMP1', 01,0);
INSERT INTO RESERVA_PRUEBA VALUES(2, 'Reserva VIP2:(AMIGOS)', '15-mar-18 9:12:00',15, 0002, 3,'EMP2', 3,0);

CREATE OR REPLACE TRIGGER ACTUALIZA_SUBTOTAL
BEFORE INSERT OR DELETE OR UPDATE ON RESERVA_PRUEBA
FOR EACH ROW
BEGIN
  IF INSERTING THEN
  :NEW.SUB_TOTAL:=:NEW.MONTO * :NEW.HORAS;
  END IF;
  IF UPDATING THEN
    IF :OLD.HORAS < :NEW.HORAS THEN
      :NEW.SUB_TOTAL:=:NEW.MONTO * :NEW.HORAS;
    ELSE
      RAISE_APPLICATION_ERROR (-20999, 'Horas Añadidas no pueden ser menor a las ya consumidas');
    END IF;
  END IF;
END;

INSERT INTO RESERVA_PRUEBA VALUES(3, 'Reserva para:1 persona', '15-nov-18 8:12:12',10, 4,2, 'EMP2',2,0);

UPDATE RESERVA_PRUEBA SET HORAS = 3 WHERE COD_RESERVA = 2;
UPDATE RESERVA_PRUEBA SET HORAS = 2 WHERE COD_RESERVA = 2;