Create Trigger That Prevent Order Being Processed If The Amount Is 0 In Storage

When i process a order in table Orders_t i would like to check in storage_t if we defenetly have it in storage. .... if we have it in storage, i decrease the "amount" by 1 ..(amount -1), and process the order. Otherwise it will return nothing.

This is what i´ve come up with so far:


CREATE TRIGGER checkInStorage
ON orders_t
FOR INSERT, UPDATE

AS
DECLARE
@tOrderId char(3),
BEGIN
SET @tOrderId = (SELECT orderId FROM INSERTED)

--check if the amount in storage

IF EXIST(SELECT amount FROM storage_t WHERE orderId = @tOrderId and amount >= 0)
BEGIN --if it return true, i update the storage by decrease the amount with one
UPDATE storage_t
SET (amount = amount - 1)
WHERE orderId = @tOrderId
END




this doesn´t work...