整理一下思路

1

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

2

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