select c.cust_name,o.order_num,
sum(oi.quantity*oi.item_price) as OrderTotal
from Customers as c
inner join Orders as o
on c.cust_id = o.cust_id

inner join OrderItems as oi
on o.order_num = oi.order_num

group by c.cust_name,oi.order_num
order by c.cust_name,oi.order_num asc

昨天刷了40多道题，今天再刷好想吐，脑子嗡嗡的