因为表中没有total_ordered,我们先使用sum构造出来，然后两个表做内连接Join..on条件，连接条件是order_num.desc表示降序排列。 以下是代码：

select cust_id,total_ordered
from Orders o
join
(select 
 cust_num,sum(order_item *quantity) total_ordered
from OrderItms
group by order_num
)t
on o.order_num = t.order_num
order by total_ordered desc;