select cust_name,t1.order_num,sum(quantity*item_price) as OrderTotal
from Customers t inner join Orders t1 on t.cust_id = t1.cust_id
inner join OrderItems  t2 on t1.order_num = t2.order_num
group by cust_name,t1.order_num
order by cust_name,t1.order_num

易错点：
1、因为order_num在多个表出现，所以在引用列的时候，需要注明该列来自哪个表。
2、忘记使用group by，以及忘记排序