只能说：简单易懂

将查询第一个表的结果组合成第三个表

通过order_num联结第二个表和第三个表

select cust_id,total_ordered
from Orders,(
   select order_num,sum(item_price*quantity) as total_ordered
    from OrderItems
    group by order_num
    order by total_ordered
    )as tot 
where Orders.order_num=tot.order_num
order by total_ordered desc