導(dǎo)讀:1建站知識統(tǒng)計(jì)每個(gè)城市的最新10個(gè)產(chǎn)品本文采用了游標(biāo)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網(wǎng)站建設(shè)網(wǎng)站建設(shè)多少錢。

有產(chǎn)品表，包含網(wǎng)站seo優(yōu)化診斷工具id，name，city，addtime四個(gè)字段，因報(bào)表需要按城市分組，統(tǒng)計(jì)每個(gè)城市的最新10個(gè)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(fù)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(fù)制代碼 代碼如下:

select no, id,name,city from (select no =row_number() over (partition by city order by addtimseo網(wǎng)站優(yōu)化e 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(fù)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(shí)，取消任務(wù)執(zhí)行了；第二次執(zhí)行到13分鐘時(shí)，沒有hold住又直接停止了，實(shí)在無法忍受。

復(fù)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(biāo)方法，這個(gè)最后測試的，執(zhí)行了5次，每次都是10秒完成，感覺還不錯(cuò)。

復(fù)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 網(wǎng)站建設(shè)教程mycursor Select * from #Top order by city asc,addtime desc drop table #Top

通過上述對比不難發(fā)現(xiàn)，在面臨Group獲取Top N場景時(shí)，可以首選row_number，游標(biāo)cursor其次，另外兩個(gè)就基本不考慮了，數(shù)據(jù)量大的時(shí)候根本沒法使用。相關(guān)網(wǎng)站建設(shè)網(wǎng)站建設(shè)多少錢。

關(guān)鍵詞標(biāo)簽: 標(biāo)簽 SQL 首選