mysql sum

SELECT
    `order_descriptions`.`tanggal_order`, `order_descriptions`.`quantity`, `products`.`name`, `products`.`price`, `units`.`satuan`, `units`.`pangkat`
    , order_descriptions.quantity*products.price AS harga
        FROM `orders`
            inner join `order_descriptions` on `orders`.`id` = `order_descriptions`.`order_id`
                inner join `products` on `products`.`id` = `orders`.`product_id`
                    inner join `units` on `units`.`id` = `products`.`unit`
                        WHERE `orders`.`invoice_id` = '4'
                            ORDER BY `orders`.`id` DESC