SQL中Group分组获取Top N 方法实现
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:
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,执行5次,平均下来8秒左右,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。
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。
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。
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。