SQL中Group分組獲得Top N辦法完成可首選row_number。本站提示廣大學習愛好者：（SQL中Group分組獲得Top N辦法完成可首選row_number）文章只能為提供參考，不一定能成為您想要的結果。以下是SQL中Group分組獲得Top N辦法完成可首選row_number正文
有產物表，包括id，name，city，addtime四個字段，因報表須要按城市分組，統計每一個城市的最新10個產物，便向該表中拔出了100萬數據，做了以下系列測試：
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、采取row_number辦法，履行5次，均勻上去8秒閣下，速度最快。
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、采取cross apply辦法，履行了3次，根本都在3分5秒以上，曾經很慢了。
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、采取Count查詢，只履行了兩次，第一次履行到5分鐘時，撤消義務履行了；第二次履行到13分鐘時，沒有hold住又直接停滯了，其實沒法忍耐。
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、采取游標辦法，這個最初測試的，履行了5次，每次都是10秒完成，感到還不錯。
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
經由過程上述比較不難發明，在面對Group獲得Top N場景時，可以首選row_number，游標cursor其次，別的兩個就根本不斟酌了，數據量年夜的時刻基本沒法應用。
