Trigger after UPDATE

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

	RETURN NULL;
END;

$$ LANGUAGE plpgsql;

CREATE TRIGGER after_sale_update
AFTER UPDATE ON sales
FOR EACH ROW
EXECUTE FUNCTION update_product_quantity()