玩转数据库之 Group by Grouping

有的时候咱们要从数据库里把数据组织成树结构再展示到页面上

像下面这样

今天咱们用Group 和Grouping实现它，并总结一下它俩。

先看一下概念,再用代码一点一点去理解它们，最后我会给出完整的代码

Group By : 语句用于结合合计函数，根据一个或多个列对结果集进行分组。

Grouping :指示是否聚合 GROUP BY 列表中的指定列表达式。 在结果集中，若是 GROUPING 返回 1 则指示聚合；

                返回 0 则指示不聚合。 若是指定了 GROUP BY，则 GROUPING 只能用在 SELECT <select> 列表、HAVING 和 ORDER BY 子句中。

ROLLUP ：生成简单的 GROUP BY 聚合行以及小计行或超聚合行，还生成一个总计行。

让咱们先建一个数据库，并添加一些数据

use master  
go 
if exists(select 1 from sysdatabases where name ='MyGroupDB')
	ALTER DATABASE MyGroupDB SET SINGLE_USER with ROLLBACK IMMEDIATE  
	drop database MyGroupDB
go

create database MyGroupDB
go
use MyGroupDB
go

create Table Category
(
	Category_ID int identity(1,1),
	Category_Name varchar(100)
)
go
create Table Product
(
	Product_ID int identity(1,1),
	CategoryID int ,
	Product_Name varchar(100)
)
go
insert into Category values('手机')
insert into Category values('台式机')
insert into Category values('数码相机')
go

insert into Product values(1,'诺基亚')
insert into Product values(1,'三星')
insert into Product values(1,'苹果')

insert into Product values(2,'HP')
insert into Product values(2,'IBM')
insert into Product values(2,'Dell')


insert into Product values(3,'佳能')
insert into Product values(3,'尼康')
insert into Product values(3,'索尼')
go

看一下它们的数据

select *  
from Category 
left join Product on Category_ID = CategoryID

咱们把它们用Group By分一下组

select Category_ID ,                
        Category_Name,        
        CategoryID,        
        Product_Name    
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,CategoryID,Category_Name,Product_Name

咱们看到这样和没有分组时展示的数据是同样的，让咱们加上 ROLLUP 加上合计行

select Category_ID ,				
		Category_Name,		
		CategoryID,		
		Product_Name	
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,CategoryID,Category_Name,Product_Name  with rollup

咱们看到了好多NULL数据，并且颇有规律

这些规律咱们能够用Grouping 看到

select Category_ID ,
		GROUPING(Category_ID) as Category_IDGP,						
		Category_Name,
		GROUPING(Category_Name) as Category_NameGP,		
		CategoryID,
		GROUPING(CategoryID) as CategoryIDGP,		
		Product_Name,
		GROUPING(Product_Name) as Product_NameGP
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,Category_Name,CategoryID,Product_Name with rollup

 你会发现那些Null值就是Grouping 为1的时候

最后一行的合计是Categrory_ID的，咱们不须要，CategoryID的合计咱们也不须要咱们要怎么去掉它们呢，在having 里

select Category_ID ,
		GROUPING(Category_ID) as Category_IDGP,		
		CategoryID,
		GROUPING(CategoryID) as CategoryIDGP,		
		Category_Name,
		GROUPING(Category_Name) as Category_NameGP,
		Product_Name,
		GROUPING(Product_Name) as Product_NameGP
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,Category_Name,CategoryID,Product_Name with rollup
having GROUPING(Category_ID)=0  and GROUPING(CategoryID)=0 

这样的结果 咱们看到只有Product_Name的Grouping有为1 了

咱们就是用它去实现这棵树

select 
case GROUPING(Product_Name) when 1 then Category_Name  else '' end as Category_Name,
case GROUPING(Product_Name) when 0 then Product_Name else '' end as Product_Name
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,Category_Name,CategoryID,Product_Name with rollup
having GROUPING(Category_ID)=0  and GROUPING(CategoryID)=0 
order by Category_ID ,Product_Name

下面是完整的代码

use master  
go 
if exists(select 1 from sysdatabases where name ='MyGroupDB')
	ALTER DATABASE MyGroupDB SET SINGLE_USER with ROLLBACK IMMEDIATE  
	drop database MyGroupDB
go

create database MyGroupDB
go
use MyGroupDB
go

create Table Category
(
	Category_ID int identity(1,1),
	Category_Name varchar(100)
)
go
create Table Product
(
	Product_ID int identity(1,1),
	CategoryID int ,
	Product_Name varchar(100)
)
go
insert into Category values('手机')
insert into Category values('台式机')
insert into Category values('数码相机')
go

insert into Product values(1,'诺基亚')
insert into Product values(1,'三星')
insert into Product values(1,'苹果')

insert into Product values(2,'HP')
insert into Product values(2,'IBM')
insert into Product values(2,'Dell')


insert into Product values(3,'佳能')
insert into Product values(3,'尼康')
insert into Product values(3,'索尼')
go


select *  
from Category 
left join Product on Category_ID = CategoryID
--------------------------------------------------------

select Category_ID ,				
		Category_Name,		
		CategoryID,		
		Product_Name	
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,CategoryID,Category_Name,Product_Name  with rollup

--------------------------------------------------------
select Category_ID ,
		GROUPING(Category_ID) as Category_IDGP,						
		Category_Name,
		GROUPING(Category_Name) as Category_NameGP,		
		CategoryID,
		GROUPING(CategoryID) as CategoryIDGP,		
		Product_Name,
		GROUPING(Product_Name) as Product_NameGP
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,Category_Name,CategoryID,Product_Name with rollup

----------------------
select Category_ID ,
		GROUPING(Category_ID) as Category_IDGP,		
		CategoryID,
		GROUPING(CategoryID) as CategoryIDGP,		
		Category_Name,
		GROUPING(Category_Name) as Category_NameGP,
		Product_Name,
		GROUPING(Product_Name) as Product_NameGP
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,Category_Name,CategoryID,Product_Name with rollup
having GROUPING(Category_ID)=0  and GROUPING(CategoryID)=0 

-------------------------

select 
case GROUPING(Product_Name) when 1 then Category_Name  else '' end as Category_Name,
case GROUPING(Product_Name) when 0 then Product_Name else '' end as Product_Name
from Category 
left join Product on Category_ID = CategoryID
group by Category_ID ,Category_Name,CategoryID,Product_Name with rollup
having GROUPING(Category_ID)=0  and GROUPING(CategoryID)=0 
order by Category_ID ,Product_Name