SQL 分组查询取前N条

平手友梨奈

一、创建测试数据

create table tb(name varchar(10),val int,memo varchar(20)) ;
insert into tb values('a', 2, 'a2(a的第二个值)') ;
insert into tb values('a', 1, 'a1--a的第一个值') ;
insert into tb values('a', 3, 'a3:a的第三个值') ;
insert into tb values('b', 1, 'b1--b的第一个值') ;
insert into tb values('b', 3, 'b3:b的第三个值') ;
insert into tb values('b', 2, 'b2b2b2b2') ;
insert into tb values('b', 4, 'b4b4') ;
insert into tb values('b', 5, 'b5b5b5b5b5') ;
go;

二、按name分组取val最大的值所在行的数据

--方法1：select a.* from tb a where val = (select max(val) from tb where name = a.name) order by a.name 
--方法2： 
select a.* from tb a where not exists(select 1 from tb where name = a.name and val > a.val) 
--方法3： 
select a.* from tb a,(select name,max(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by a.name 
--方法4： 
select a.* from tb a inner join (select name , max(val) val from tb group by name) b on a.name = b.name and a.val = b.val order by a.name 
--方法5 
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name 
/* 
name val memo 
---------- ----------- -------------------- 
a 3 a3:a的第三个值 
b 5 b5b5b5b5b5 

*/

本人推荐使用1,3,4,结果显示1,3,4效率相同，2，5效率差些，不过我3,4效率相同毫无疑问，1就不一样了，想不搞了。

三、按name分组取val最小的值所在行的数据。

--方法1：select a.* from tb a where val = (select min(val) from tb where name = a.name) order by a.name 
--方法2： 
select a.* from tb a where not exists(select 1 from tb where name = a.name and val < a.val) 
--方法3： 
select a.* from tb a,(select name,min(val) val from tb group by name) b where a.name = b.name and a.val = b.val order by a.name 
--方法4： 
select a.* from tb a inner join (select name , min(val) val from tb group by name) b on a.name = b.name and a.val = b.val order by a.name 
--方法5 
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val < a.val) order by a.name 
/* 
name val memo 
---------- ----------- -------------------- 
a 1 a1--a的第一个值 
b 1 b1--b的第一个值 
*/

四、按name分组取最小的两个(N个)val

select a.* from tb a where 2 > (select count(*) from tb where name = a.name and val < a.val ) order by a.name,a.val

select a.* from tb a where val in (select top 2 val from tb where name=a.name order by val) order by a.name,a.val 

select a.* from tb a where exists (select count(*) from tb where name = a.name and val < a.val having Count(*) < 2) order by a.name 
/* 
name val memo 
---------- ----------- -------------------- 
a 1 a1--a的第一个值 
a 2 a2(a的第二个值) 
b 1 b1--b的第一个值 
b 2 b2b2b2b2 

*/

五、按name分组取最大的两个(N个)val

select a.* from tb a where 2 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name,a.val 

select a.* from tb a where val in (select top 2 val from tb where name=a.name order by val desc) order by a.name,a.val 

select a.* from tb a where exists (select count(*) from tb where name = a.name and val > a.val having Count(*) < 2) order by a.name 
/* 
name val memo 
---------- ----------- -------------------- 
a 2 a2(a的第二个值) 
a 3 a3:a的第三个值 
b 4 b4b4 
b 5 b5b5b5b5b5 
*/