题解 | #返回每个顾客不同订单的总金额#

法一：
select cust_id, total_order
from (select order_num, sum(quantity * item_price) as total_order
    from OrderItems
    group by order_num) t
inner join Orders o1
on o1.order_num = t.order_num
order by total_order desc
法二：
select cust_id, sum(quantity * item_price) as total_order
from OrderItems o1, Orders o2
where o1.order_num = o2.order_num
group by cust_id
order by total_order desc