有的时候我们要从数据库里把数据组织成树结构再展现到页面上
像下面这样
今天我们用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，如果 GROUPING 返回 1 则指示聚合；
返回 0 则指示不聚合。 如果指定了 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。
让我们先建一个数据库，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 MyGroupDBgocreate database MyGroupDBgouse MyGroupDBgocreate Table Category( Category_ID int identity(1,1), Category_Name varchar(100))gocreate Table Product( Product_ID int identity(1,1), CategoryID int , Product_Name varchar(100))goinsert into Category values(‘手机‘)insert into Category values(‘台式机‘)insert into Category values(‘数码相机‘)goinsert 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 |
我们把它们用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
我们看到这样和没有分组时展现的数据是一样的，让我们加上 ROLLUP 加上合计行
select Category_ID , Category_Name, CategoryID, Product_Name from Category left join Product on Category_ID = CategoryIDgroup by Category_ID ,CategoryID,Category_Name,Product_Name with rollup |
我们看到了好多NULL数据，而且很有规律
这些规律我们可以用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_NameGPfrom Category left join Product on Category_ID = CategoryIDgroup 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，我们不需要，CategoryID的合计我们也不需要我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_NameGPfrom Category left join Product on Category_ID = CategoryIDgroup by Category_ID ,Category_Name,CategoryID,Product_Name with rolluphaving GROUPING(Category_ID)=0 and GROUPING(CategoryID)=0 |
这样的结果 我们看到只有Product_Name的Grouping有为1 了
我们就是用它去实现这棵树
selectcase 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_Namefrom Category left join Product on Category_ID = CategoryIDgroup by Category_ID ,Category_Name,CategoryID,Product_Name with rolluphaving 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 MyGroupDBgocreate database MyGroupDBgouse MyGroupDBgocreate Table Category( Category_ID int identity(1,1), Category_Name varchar(100))gocreate Table Product( Product_ID int identity(1,1), CategoryID int , Product_Name varchar(100))goinsert into Category values(‘手机‘)insert into Category values(‘台式机‘)insert into Category values(‘数码相机‘)goinsert 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‘)goselect * 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 = CategoryIDgroup 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_NameGPfrom Category left join Product on Category_ID = CategoryIDgroup 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_NameGPfrom Category left join Product on Category_ID = CategoryIDgroup by Category_ID ,Category_Name,CategoryID,Product_Name with rolluphaving GROUPING(Category_ID)=0 and GROUPING(CategoryID)=0 -------------------------selectcase 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_Namefrom Category left join Product on Category_ID = CategoryIDgroup by Category_ID ,Category_Name,CategoryID,Product_Name with rolluphaving GROUPING(Category_ID)=0 and GROUPING(CategoryID)=0 order by Category_ID ,Product_Name |
原文：http://www.cnblogs.com/lcword/p/5719288.html