Trigger after DELETE

CREATE OR REPLACE FUNCTION restore_product_quantity() 
RETURNS TRIGGER AS $$
BEGIN
	UPDATE products
	SET quantity = quantity + OLD.quantitysold
	WHERE productid = OLD.productid;

	RETURN NULL;
END;

$$LANGUAGE plpgsql;

CREATE TRIGGER after_sales_delete
AFTER DELETE ON sales
FOR EACH ROW
EXECUTE FUNCTION restore_product_quantity()