MySQL触发器插入/更新

Question

I hope you can help me with a trigger.

I have two tables Books and Order_Details . I want the trigger to update InStock whenever a book from the table Books with the same OrderID is sold (Quantity)

Can you help me finish the trigger
All i have now is the following:

CREATE TRIGGER UpdateInStock 
    BEFORE insert ON Order_Details
    FOR EACH ROW 
BEGIN
    Update Books
    SET action = 'update',
        BookID = BookID,
    .....

Answer 1

Just call a simple UPDATE statement, to subtract the InStock value using New.Quantity .

Check the following:

DELIMITER $$
DROP TRIGGER IF EXISTS UpdateInStock $$
CREATE TRIGGER UpdateInStock 
    BEFORE insert ON Order_Details
    FOR EACH ROW 
BEGIN
    Update Books
    SET InStock = InStock - New.QUANTITY 
    WHERE BookID = New.BookID;
END $$
DELIMITER ;