SQL中Group分組獲取Top N方法實現(xiàn)可首選row_number

更新時間：2013年03月10日 17:42:51 作者：

統(tǒng)計每個城市的最新10個產(chǎn)品本文采用了游標方法/Count查詢/cross apply方法/row_number方法等等對比不難發(fā)現(xiàn)Group獲取Top N場景時，可以首選row_number，游標cursor其次，另外兩個就基本不考慮了

有產(chǎn)品表，包含id，name，city，addtime四個字段，因報表需要按城市分組，統(tǒng)計每個城市的最新10個產(chǎn)品，便向該表中插入了100萬數(shù)據(jù)，做了如下系列測試：

復制代碼代碼如下:

 
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，執(zhí)行5次，平均下來8秒左右，速度最快。

復制代碼代碼如下:

 
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，執(zhí)行了3次，基本都在3分5秒以上，已經(jīng)很慢了。

復制代碼代碼如下:

 
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查詢，只執(zhí)行了兩次，第一次執(zhí)行到5分鐘時，取消任務執(zhí)行了；第二次執(zhí)行到13分鐘時，沒有hold住又直接停止了，實在無法忍受。

復制代碼代碼如下:

 
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、采用游標方法，這個最后測試的，執(zhí)行了5次，每次都是10秒完成，感覺還不錯。

復制代碼代碼如下:

 
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 