先子查询，得到每个订单的总金额 再与orders表连接，得到每个顾客的不同订单的总金额
select cust_id,sum(a)
from Orders
join 
(select order_num,sum(item_price*quantity) as a
from OrderItems
group by order_num) as b
on Orders.order_num = b.order_num
group by cust_id
order by sum(a) desc