题解 | 返回每个顾客不同订单的总金额

/* 方法1：用符合直觉的连表实现 
SELECT
    cust_id,
    SUM(oi.item_price * oi.quantity) AS total_ordered
FROM
    Orders o
INNER JOIN
    OrderItems oi ON oi.order_num = o.order_num
GROUP BY
    cust_id
ORDER BY
    total_ordered DESC
;
*/

/* 方法2：使用子查询的方式 */
SELECT
    cust_id,
    (
        SELECT
            SUM(item_price * quantity)
        FROM
            OrderItems oi
        WHERE
            oi.order_num = o.order_num
    ) AS total_ordered
FROM
    Orders o
ORDER BY
    total_ordered DESC
;