How to update a rows with same prodID on a UPDATE? - mysql

Question

I get the following error:
"Can't update table 'Product' in stored function/trigger because it is already used by statement which invoked this stored function/trigger."
CREATE TRIGGER `SalesDB`.`updateSum` AFTER UPDATE ON SalesDB.Product FOR EACH ROW
BEGIN
DECLARE totalStock INT;
SELECT SUM(stock)
INTO totalStock
FROM Product
WHERE Product.prodID= NEW.prodID;
UPDATE Product SET StockSum = totalStock
WHERE prodID = NEW.prodID;
END;
I understand that there is a recursive issue, but how do I do something like this? Is there a way to use triggers to do this kind of thing? Should this be done in a Stored Procedure instead, and how do I run the stroed procedure on a Update Product event?
I need to update the stock of a product and then go thru all rows that have the same prodID calculate the SUM and Update the SumTotal field of all the rows.

Answer 1

UPDATED Unfortunately you can't do this in MySql due to the limitations of triggers implementation.
Possible options are:
Create stored procedure and call it after each update on Product table
Store stockSum in a separate table (e.g. ProductStock), use triggers (INSERT, UPDATE) to update stockSum values, create a view on joined Product and ProductStock
Use MySql events or a cron job to update stockSum in Product table if some delay is acceptable
IMHO best approach is 2 separating Product and ProductStock tables:
ProductStock might look like this
CREATE TABLE ProductStock
(
prodID int NOT NULL PRIMARY KEY,
stockSum decimal(12, 3)
);
Now triggers
CREATE TRIGGER tg_product_update
AFTER UPDATE ON Product
FOR EACH ROW
CALL sp_updateProductStock(NEW.prodID);
CREATE TRIGGER tg_product_insert
AFTER INSERT ON Product
FOR EACH ROW
CALL sp_updateProductStock(NEW.prodID);
and a stored procedure
CREATE PROCEDURE sp_updateProductStock(IN ProductID INT)
REPLACE INTO ProductStock (prodID, stockSum)
SELECT ProductID,
(SELECT SUM(COALESCE(stock, 0))
FROM Product
WHERE prodID = ProductID);
And finally a view
CREATE VIEW vw_Product AS
SELECT p.id, p.prodID, ... , p.stock, s.stockSum
FROM Product p JOIN ProductStock s
ON p.prodID = s.prodID
Here is SQLFiddle example