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SQL中Group分组获取Top N方法实现可首选row_number

2019-01-03 MsSQL 前端之家

前端之家收集整理的这篇文章主要介绍了SQL中Group分组获取Top N方法实现可首选row_number，前端之家小编觉得挺不错的，现在分享给大家，也给大家做个参考。

有产品表，包含id，name，city，addtime四个字段，因报表需要按城市分组，统计每个城市的最新10个产品，便向该表中插入了100万数据，做了如下系列测试：
<div class="codetitle"><a style="CURSOR: pointer" data="26601" class="copybut" id="copybut26601" onclick="doCopy('code26601')"> 代码如下:

<div class="codebody" id="code26601">
CREATE TABLE [dbo].[products](
[id] [int] IDENTITY(1,1) NOT NULL,
[name] nvarchar NULL,
[addtime] [datetime] NULL,
[city] nvarchar 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。
<div class="codetitle"><a style="CURSOR: pointer" data="18315" class="copybut" id="copybut18315" onclick="doCopy('code18315')"> 代码如下:

<div class="codebody" id="code18315">
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。
<div class="codetitle"><a style="CURSOR: pointer" data="96289" class="copybut" id="copybut96289" onclick="doCopy('code96289')"> 代码如下:

<div class="codebody" id="code96289">
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。
<div class="codetitle"><a style="CURSOR: pointer" data="35810" class="copybut" id="copybut35810" onclick="doCopy('code35810')"> 代码如下:

<div class="codebody" id="code35810">
select id,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。
<div class="codetitle"><a style="CURSOR: pointer" data="17247" class="copybut" id="copybut17247" onclick="doCopy('code17247')"> 代码如下:<div class="codebody" id="code17247">
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,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

通过上述对比不难发现，在面临Group获取Top N场景时，可以首选row_number，游标cursor其次，另外两个就基本不考虑了，数据量大的时候根本没法使用。

原文链接：https://www.f2er.com/mssql/63572.html

row_number