SELECT a.cust_id, SUM(b.item_price*b.quantity) total_ordered 
FROM Orders a, OrderItems b
WHERE a.order_num = b.order_num
GROUP BY a.cust_id
ORDER BY total_ordered DESC;

题解：

1.任务：计算每个顾客的所有订单的总金额

思路：

1.计算订单总金额：字段相乘和求和函数

2.不同顾客，分组语句

3.表的自连接，where语句