题解 | 返回顾客名称和相关订单号以及每个订单的总价

select cust_name,a.order_num,sum(quantity*item_price) as OrderTotal
from OrderItems a
left join Orders b using(order_num)
left join Customers c using(cust_id)
group by cust_name,a.order_num
order by cust_name,a.order_num

三表关联，按照客户姓名和订单名聚合，使用sum对每个小订单额（数量*价格）求和