Trigger to decrease a counter - mysql

Question

I want to create a trigger to decrease the quantity of available items after an order has been made.
I tried to create a trigger. I'm using phpMyAdmin, setting trigger AFTER INSERT in the table products_in_order
SET #qtt = (
SELECT quantity
FROM products_in_order
ORDER BY inorder_id DESC
LIMIT 1)
SET #code = (
SELECT product_code
FROM products_in_order
ORDER BY inorder_id DESC
LIMIT 1)
UPDATE products
SET quantity = quantity-#qtt
WHERE product_code=#code
Why does the first SET work, and the second, too, but as soon as I write them both, i get an error?
How to make this trigger work correctly?
Sample data:
products represents all products available in the shop, and has columns:
product_code | name | price | quantity
product_in_order represents a set of products of the same type, that can be added to order. It has colums:
inorder_id | product_code | price | order_no
inorder_id | product_code | price | quantity | order_no
When somebody adds a set of products to the order, I want the overall quantity of that product to decrease.

Answer 1

You don't need to join back to the newly inserted / updated table* - just use the New pseudo row from within your trigger, which already has the values you need to join into the master Product table:
CREATE TRIGGER `MyTrigger`
AFTER INSERT ON `products_in_order`
FOR EACH ROW
BEGIN
UPDATE products
SET quantity = quantity-New.quantity
WHERE product_code=New.product_code;
END;
SqlFiddle here demonstrating that the starting value of Product is depleted by the amount inserted into products_in_order for the applicable product code.
* Assuming inorder_id is an AUTO_INCREMENT, selecting from this table twice without any locking considerations will also be prone to concurrency issues, e.g. if another writer inserts another row while your trigger is executing. Another good reason to use New