题解 | 返回每个顾客不同订单的总金额
SELECT cust_id, ( SELECT SUM(item_price*quantity) FROM OrderItems WHERE OrderItems.order_num=Orders.order_num ) AS total_ordered FROM Orders ORDER BY total_ordered DESC;
SUM()计算满足WHERE条件的列值之和
SELECT cust_id, ( SELECT SUM(item_price*quantity) FROM OrderItems WHERE OrderItems.order_num=Orders.order_num ) AS total_ordered FROM Orders ORDER BY total_ordered DESC;
SUM()计算满足WHERE条件的列值之和
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