mysql group by分组取每组前几条记录加order by排序方法

/* 
数据如下： 
name val memo 
a 2 a2(a的第二个值) 
a 1 a1--a的第一个值 
a 3 a3:a的第三个值 
b 1 b1--b的第一个值 
b 3 b3:b的第三个值 
b 2 b2b2b2b2 
b 4 b4b4 
b 5 b5b5b5b5b5 
*/ 

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 

--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 
*/ 

--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 
*/ 

select a.* from tb a where val = (select top 1 val from tb where name = a.name) order by a.name 
/* 
name val memo 
---------- ----------- -------------------- 
a 2 a2(a的第二个值) 
b 1 b1--b的第一个值 
*/ 

select a.* from tb a where val = (select top 1 val from tb where name = a.name order by newid()) order by a.name/* 
name val memo 
---------- ----------- -------------------- 
a 1 a1--a的第一个值 
b 5 b5b5b5b5b5 
*/ 

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

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 
*/ 

/* 
数据如下： 
name val memo 
a 2 a2(a的第二个值) 
a 1 a1--a的第一个值 
a 1 a1--a的第一个值 
a 3 a3:a的第三个值 
a 3 a3:a的第三个值 
b 1 b1--b的第一个值 
b 3 b3:b的第三个值 
b 2 b2b2b2b2 
b 4 b4b4 
b 5 b5b5b5b5b5 
*/ 

select * from test group by category_id order by `date`

select ... from... where.... group by... having... order by..

from... where...group by... having.... select ... order by...

select group_concat(id order by `date` desc) from `test` group by category_id

select * from `test` where id in(select SUBSTRING_INDEX(group_concat(id order by `date` desc),',',1) from `test` group by category_id ) order by `date` desc

select * from (select * from `test` order by `date` desc) `temp`  group by category_id order by `date` desc