select cust_id, sumPrice from Orders a,
(select order_num, sum(item_price*quantity) as sumPrice from OrderItems group by order_num) b where a.order_num=b.order_num order by sumPrice desc;

先从OrderItems表把订单号和价格计算出来，然后去Orders表把每个cust_id对应的order_num和OrderItems表的订单号做一个连接查询