何时使用ROLLBACK，BEGIN TRANSACTION和COMMIT

Question

我不确定我是否正确使用回滚/提交;有人能够解释如何在python中使用它们吗？

    PROCEDURE bookCar(  class IN INT,  from_date IN DATE,  to_date IN DATE, reservation_no OUT INT ) 
    DECLARE 
     available_car INT; 
    BEGIN 

    TRANSACTION

     SELECT car_id INTO  :available_car 
       FROM Cars 
      WHERE class= :class AND booked = FALSE 
      LIMIT 1; 

    COMMIT

     IF (  :available_car IS NOT NULL ) THEN BEGIN 

     TRANSACTION

         UPDATE Cars 
            SET booked = TRUE 
          WHERE car_id =  :available_car; 

--

         SELECT MAX(reservation_no)+1 INTO  :reservation_no 
           FROM Bookings; 
--         

         INSERT INTO Bookings VALUES( :reservation_no, :available_car, :from_date, :to_date ); 

    ROLLBACK

     ELSE 

    COMMIT

     END IF; 

    ROLLBACK

     END; 

我很感激我能得到的任何帮助，因为我真的很挣扎！谢谢！