select cust_name
,t2.order_num
,sum(quantity * item_price) as OrderTotal 
/*窗口函数往往适用于明细与汇总，只保留一条总计的用group by*/
from Customers t1
join Orders t2 on t1.cust_id = t2.cust_id
join OrderItems t3 on t2.order_num = t3.order_num
group by cust_name
,t2.order_num
order by cust_name