题解 | #返回每个顾客不同订单的总金额#

select t.cust_id,SUM(quantity*item_price) as total_ordered
from Orders t inner join OrderItems t1 on t.order_num=t1.order_num
group by cust_id
ORDER BY total_ordered DESC

仍然是表链接更简单易理解。

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;

关联子查询就需要理解他的运算逻辑了，认知难度更高一点，主要是实际中用的上，很难记得牢固