通过提示使用sum进行总订单额的计算，并将结果作为表带入到查询中进行排序筛选

select 
cust_id,
t.total_ordered 
from 
(select 
sum(item_price*quantity) total_ordered,
order_num 
from OrderItems 
group by order_num) t join Orders o 
on t.order_num=o.order_num
order by t.total_ordered desc