题解 | #返回每个顾客不同订单的总金额#

首先发现需要返回cust_id和total_order。而total_order是需要使用函数计算的。

故，

1. 主查询从Orders表中进行查询得到cust_id和order_num
2. 子查询针对主查询中得到的每一条记录，去OrderItems表中找，与order_num相等的行。
3. 子查询中计算结果行，得到total_ordered。

SELECT
    cust_id,
    (
        SELECT
            SUM(item_price * quantity)
        FROM
            OrderItems
        WHERE
            OrderItems.order_num = Orders.order_num
    ) total_ordered
FROM
    Orders
ORDER BY
    total_ordered DESC;