有產品表，包含id，name，city，addtime四個字段，因報表需要按城市分組，統計每個城市的最新10個產品，便向該表中插入了100萬數據，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，執行了3次，基本都在3分5秒以上，已經很慢了。
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，只執行了兩次，第一次執行到5分鐘時，取消任務執行了；第二次執行到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，這個最後測試的，執行了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
通過上述對比不難發現，在面臨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，數據量大的時候根本沒法使用。
