select o.cust_id, 
    (
        select sum(q.item_price*q.quantity)
        from OrderItems q
        where q.order_num = o.order_num
    ) total_order
from Orders o
order by total_order desc
#主查询中的total_order列将子查询的结果显示为主查询的一部分。故结果集中的每一行都有相应的cust_id和计算出的total_order。