sqlServer 取每组的前几条数据

首先的建表语句：

 if exists (select * from sysobjects where id = OBJECT_ID('[test]') and OBJECTPROPERTY(id, 'IsUserTable') = ) DROP TABLE [test]
 CREATE TABLE [test] (  [id] [int] IDENTITY (, ) NOT NULL , [name] [nvarchar] () NULL ,  [votenum] [int] NULL , [type] [nvarchar] () NULL )
 ALTER TABLE [test] WITH NOCHECK ADD CONSTRAINT [PK_test] PRIMARY KEY  NONCLUSTERED ( [id] )

 SET IDENTITY_INSERT [test] ON

 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '嶂石岩' ,  , '风景' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '云梦山' ,  , '风景' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '抱犊寨' ,  , '风景' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '崆山白云洞' ,  , '风景' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '扁鹊庙' ,  , '古迹' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '金长城' ,  , '古迹' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '避暑山庄' ,  , '古迹' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '西柏坡' ,  , '古迹' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '塞罕坝' ,  , '草原' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '草原天路' ,  , '草原' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '京北草原' ,  , '草原' )
 INSERT [test] ( [id] , [name] , [votenum] , [type] ) VALUES (  , '美林谷' ,  , '草原' )

 SET IDENTITY_INSERT [test] OFF

字段含义：name 风景区名称，votenum 景区得票数量 ，type 景区类型

实现功能：查询各分类中投票最多的两个景区，按投票数从高到低排序

实现1： ROW_NUMBER() 配合partition by 按照分组进行排序，取前两位，易懂

 select name,votenum,[type] from (
   select *,ROW_NUMBER() over(partition by [type] order by votenum desc) vn from test) b
   where b.vn<= order by [type], votenum desc

实现2：自连接

select * from test a
 where (select COUNT(*) from test b where a.type=b.type and a.votenum<b.votenum) <=
 order by [type],votenum desc

外层表a中的一条数据 在内层表b中查找的相同类型type，且b表中投票数量大于a表中该条记录的投票数量的记录数（count）,

如果小于1，说明a表中的这条数据 的投票数是最高或者是第二高的，则返回a表中的这条数据

实现3：

 select * from test a
 where a.id in(select top  b.id from test b where a.type=b.type order by b.votenum desc)
   order by [type],votenum desc