题解 | #返回每个顾客不同订单的总金额#

总思路：
1、由OrderItems表查询得到每个订单总金额(total_order)，将这个表命名为t
（但是没有顾客信息，因此需要与Order表相连接）
2、将包含每个订单总金额(total_order)的表t与包含顾客信息（cust_id）的表Orders相连接
3、筛选列cust_id, total_orderd
4、按total_orderd降序（desc）排列

#t表
select order_num,sum(item_price*quantity) as total_orderd
    from OrderItems
    group by order_num   

from Orders o
join 
	(select order_num,sum(item_price*quantity) as total_orderd
    from OrderItems
    group by order_num    
     ) t 
    using(order_num)
order by total_orderd desc