题解 | #返回每个顾客不同订单的总金额#

#1
#内连接可以，题目原因左外连接不行。
SELECT cust_id,
       SUM(quantity * item_price) AS total_ordered
FROM OrderItems ot
JOIN Orders o ON ot.order_num = o.order_num  
# FROM Orders, OrderItems
# WHERE Orders.order_num = OrderItems.order_num
GROUP BY cust_id
ORDER BY total_ordered DESC;

第二种方法： #2

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;