题解 | #返回每个顾客不同订单的总金额#（这样或许才对？）

SELECT cust_id, 
       SUM((SELECT SUM(quantity*item_price) # 新增SUM
        FROM OrderItems
        WHERE OrderItems.order_num = Orders.order_num)) AS total_ordered
FROM Orders
GROUP BY cust_id # 新增
ORDER BY total_ordered DESC;
/*
SELECT cust_id, 
       (SELECT SUM(quantity*item_price)
        FROM OrderItems
        WHERE OrderItems.order_num = Orders.order_num) AS total_ordered
FROM Orders
ORDER BY total_ordered DESC;
 # 这种做法，只是统计了表中的每一行顾客对应订单的总金额 没有考虑到多行的顾客可能是一样的（比如题目例子，有两个cust1（当然，题目例子答案的第二个cust2应该是cust1））
*/

应该在热评的基础上修改一下，使用分组查询，考虑顾客id可能一样的情况（题目例子就是）