题解 | #返回每个顾客不同订单的总金额#

难点：total_ordered 的计算

根据题目，分析解题思路：

1. 在OrderItems表，根据订单号，计算出已订购的总金额
2. 在Orders表，根据订单号，得出是哪一个顾客
3. 用order_num 列 将Orders表与OrderItems表关联

最后得出：

select a.cust_id as cust_id,b.c as total_ordered
from Orders a left join (
    select order_num,sum(item_price*quantity) as c from OrderItems group by order_num
) b on a.order_num = b.order_num order by total_ordered DESC