MySQL - How can add a new column synchronized with another table's sum of multiple columns

Question

I have to table named 'stuff' and 'given'. I wanted to add one more columnt to 'stuff' and after ALTER TABLE i used UPDATE. However this makes difference for only that moment. I mean, m new column will give the amount of stock after each given amount. 'stuff' has 'totalAmount' and given has 'amount'.

I used:

ALTER TABLE stuff ADD stock MEDIUMUNINT UNSIGNED;

UPDATE stuff SET stuff = totalAmount - (SELECT SUM(given.amount) FROM given WHERE id = given.productId);

This works for only that UPDATE. How can i make these two table so synchronized after each given amount, the stock will be affected either?

Thanks in advance :)

Answer 1

Create an trigger on INSERT for the table "given" which will update the "stuff" in table "stuff"

CREATE TRIGGER [TRIGGER_ALTER_STUFF] ON [dbo].[given] 
FOR INSERT, UPDATE
AS
BEGIN
    UPDATE stuff SET stuff = totalAmount - (SELECT SUM(given.amount) FROM given WHERE id = given.productId);
END