select
	b.`ID` AS `ID`,
	c.`product_id` AS `product_id`,
	c.`prezzo` AS `prezzo`,
	c.`disponibili` AS `quantity_max`,
	a.`ID` AS `user_id`,
	b.`quantity` AS `quantity_ask`,
	(
		SELECT
			SUM(h.prezzo * g.quantity) AS total_price_prod
		FROM
			amz_usr as f inner join amz_cart as g on f.ID = g.user_id inner join amz_prod as h on g.product_id = h.product_id
		WHERE
			c.`product_id` = g.`product_id` AND a.`ID` = g.`user_id`
	) AS `total_price_prod`
from
	amz_usr as a inner join amz_cart as b on a.ID = b.user_id inner join amz_prod as c on b.product_id = c.product_id;