MySQL Trigger Sum Two Columns where userID is the same - mysql

Question

I have the following product table
id | companyID | prodID | price | stock | stockAvailable | sumStock
1 A 2 10 5 4
2 B 2 50 10 5
I need to have a trigger when I update a product row that will update the sumStock.
I am new to Triggers, my attempt failed:
CREATE TRIGGER `SalesDB`.`stockSumUpdate` BEFORE UPDATE
ON SalesDB.Product FOR EACH ROW
BEGIN
SET Product.sumStock = Product.stock + Product.stockAvailable
END
My Goal is in this case to calculate the SUM(stock) AS stockSum Where ProductID=2
in this case that would be 15 and then add that to the sumStock. Then add the stockAvailable column to that as well. So in sumStock for both collumns I would have 24.
Result would be:
id | companyID | prodID | price | stock | stockAvailable | sumStock
1 A 2 10 5 4 29
2 B 2 50 10 5 29

Answer 1

Try this syntax:
CREATE TRIGGER `SalesDB`.`stockSumUpdate` BEFORE UPDATE
ON SalesDB.Product FOR EACH ROW
BEGIN
SET NEW.sumStock = NEW.stock + NEW.stockAvailable
END;
That adds within a row of a table.
To get the total, you would use something like:
CREATE TRIGGER `SalesDB`.`stockSumUpdate` BEFORE UPDATE
ON SalesDB.Product FOR EACH ROW
BEGIN
SET NEW.sumStock = (select sum(stock + stockAvailable)
from products where p.prodid = new.prodid and p.id <> id
) + NEW.stock + NEW.stockAvailable;
END;
Except, this still, doesn't do what you want. If you are updating multiple rows at a time, you will get different total stock values.
In other words, you are trying to update a group of rows whenever you update a single row. Gosh, when put like that, it doesn't seem like a good idea. The second set of udpates could update even more rows and so on and so on (it wouldn't happen because of the product).
Instead, create a table with product as a primary key and the stock available in that table (might be an existing table). Then update the summary table every time there is a change in this table.