SQL - 多表多分组计算求和

描述

Customers 表有字段，顾客名称：cust_name、顾客id：cust_id

cust_id	cust_name
cust10	andy
cust1	ben
cust2	tony
cust22	tom
cust221	an
cust2217	hex

Orders订单信息表，含有字段，订单号：order_num、顾客id：cust_id

order_num	cust_id
a1	cust10
a2	cust1
a3	cust2
a4	cust22
a5	cust221
a7	cust2217

OrderItems表有字段，商品订单号：order_num、商品数量：quantity、商品价格：item_price

order_num	quantity	item_price
a1	1000	10
a2	200	10
a3	10	15
a4	25	50
a5	15	25
a7	7	7

【问题】

除了返回顾客名称和订单号，返回 Customers 表中的顾客名称（cust_name）和Orders 表中的相关订单号（order_num），添加第三列 OrderTotal，其中包含每个订单的总价，并按顾客名称再按订单号对结果进行升序排序。

 

【示例结果】返回顾客名称 cust_name、订单号order_num、订单总额OrderTotal

cust_name	order_num	OrderTotal
an	a5	375
andy	a1	10000
ben	a2	2000
hex	a7	49
tom	a4	1250
tony	a3	150

【示例解析】

例如顾客名称cust_name为an的顾客的订单a5的订单总额为quantity*item_price = 15 * 25 = 375，最后以cust_name和order_num来进行升序排序。

 

 

 

我的代码：

Select c.cust_name, o.order_num, sum(i.quantity*i.item_price) as OrderTotal 
FROM Customers c 
join Orders o on c.cust_id = o.cust_id 
join OrderItems i on o.order_num = i.order_num
group by c.cust_name, o.order_num 
order by
    c.cust_name, o.order_num;

这里需要注意，group by 要先按c表的cust_name分组，然后按o表的order_num分组，不能跳过左侧的表而用最里层的i进行分组。