update multiple rows with trigger after insert (sql server) - sql

Question

I have a table orderDetails that contains the products of an order
productId
color
size
quantity
and a table stock
productId
size
color
stock
When a order is completed I use this query to insert the items in the table orderDetails
INSERT INTO orderDetail(orderId, productId, productColor, productSize, productQuantity , cost productName)
SELECT
#orderId, products_translations.id, cart.productColor, cart.productSize,
cart.productQuantity, cart.cost, products_translations.name
FROM cart
INNER JOIN products_translations ON cart.productID = products_translations.id
WHERE
(cart.cartId = #cartId) AND
(products_translations.language = 1)
Then I have a trigger on table orderDetails:
ALTER TRIGGER [dbo].[scalaProdotti]
ON [dbo].[orderDetail]
FOR INSERT
AS
DECLARE #size int
DECLARE #color char(6)
DECLARE #quantity int
DECLARE #product int
BEGIN
SELECT #size = productSize FROM inserted
SELECT #color = productColor FROM inserted
SELECT #quantity = productQuantity FROM inserted
SELECT #product = productId FROM inserted
UPDATE stock SET quantity = quantity - #quantity WHERE size=#size AND color=#color AND product=#product
END
With this trigger I want to decrease the stock, but only the first product is affected, the other quantities remain the same.
What am I missing?
Thank you.

Answer 1

The main point is: you're assuming that the trigger will be called for each row being inserted - this is not the case.
Your trigger will be called once per statement - but that statement can insert mulitple rows at once.
In such a case, the Inserted table inside the trigger will contain multiple rows and your statements:
SELECT #size = productSize FROM inserted
SELECT #color = productColor FROM inserted
SELECT #quantity = productQuantity FROM inserted
SELECT #product = productId FROM inserted
will fail or will happen to select only the first row inserted and disregard the rest of the inserts.
You need to rewrite your trigger to cope with the fact that Inserted can contain multiple inserted rows at the same time
Your code in the trigger should look something like this:
UPDATE stock
FROM Inserted i
SET
stock.quantity = quantity - i.quantity
WHERE
stock.size = i.size
AND stock.color = i.color
AND stock.product = i.product