Trigger after INSERT

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

	return NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER after_sale_insert
AFTER INSERT ON sales
FOR EACH ROW
EXECUTE FUNCTION update_product_quantity()