SQL中Group分组获取Top N方法实现可首选row

复制代码代码如下:

 
CREATE TABLE [dbo].[products]( 

[id] [int] IDENTITY(1,1) NOT NULL, 

[name] [nvarchar](50) NULL, 

[addtime] [datetime] NULL, 

[city] [nvarchar](10) NULL, 

CONSTRAINT [PK_products] PRIMARY KEY CLUSTERED 

( 

[id] ASC 

)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 

) ON [PRIMARY]

1、采用row_number方法，执行5次，平均下来8秒左右，速度最快。

复制代码代码如下:

 
select no, id,name,city 

from (select no =row_number() over (partition by city order by addtime desc), * from products)t 

where no< 11 order by city asc,addtime desc

2、采用cross apply方法，执行了3次，基本都在3分5秒以上，已经很慢了。

复制代码代码如下:

 
select distinct b.id,b.name,b.city from products a 

cross apply (select top 10 * from products where city = a.city order by addtime desc) b

3、采用Count查询，只执行了两次，第一次执行到5分钟时，取消任务执行了；第二次执行到13分钟时，没有hold住又直接停止了，实在无法忍受。

复制代码代码如下:

 
select id,name,city from products a 

where ( select count(city) from products where a.city = city and addtime>a.addtime) < 10 

order by city asc,addtime desc

4、采用游标方法，这个最后测试的，执行了5次，每次都是10秒完成，感觉还不错。

复制代码代码如下:

 
declare @city nvarchar(10) 

create table #Top(id int,name nvarchar(50),city nvarchar(10),addtime datetime) 

declare mycursor cursor for 

select distinct city from products order by city asc 

open mycursor 

fetch next from mycursor into @city 

while @@fetch_status =0 

begin 

insert into #Top 

select top 10 id,name,city,addtime from products where city = @city 

fetch next from mycursor into @city 

end 

close mycursor 

deallocate mycursor 

Select * from #Top order by city asc,addtime desc 

drop table #Top