mysql分组获取最大值，
mysql分组取最大(最小、最新、前N条)条记录
在数据库开发过程中，我们要为每种类型的数据取出前几条记录，或者是取最新、最小、最大等等，这个该如何实现呢，本文章向大家介绍如何实现mysql分组取最大(最小、最新、前N条)条记录。需要的可以参考一下。
先看一下本示例中需要使用到的数据
创建表并插入数据：
CREATE TABLE `tb` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(10) CHARACTER SET latin1 DEFAULT NULL,
`val` int(11) DEFAULT NULL,
`memo` varchar(20) CHARACTER SET latin1 DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8
insert into tb values('a', 2, 'a2');
insert into tb values('a', 1, 'a1');
insert into tb values('a', 3, 'a3');
insert into tb values('b', 1, 'b1');
insert into tb values('b', 3, 'b3');
insert into tb values('b', 2, 'b2');
insert into tb values('b', 4, 'b4');
insert into tb values('b', 5, 'b5');
数据表如下：
| name | val | memo |
|---|---|---|
| a | 2 | a2 |
| a | 1 | a1 |
| a | 2 | a3 |
| b | 1 | b1 |
| b | 3 | b3 |
| b | 2 | b2 |
| b | 4 | b4 |
| b | 5 | b5 |
按name分组取val最大的值所在行的数据
方法一：
select a.* from tb a where val = (select max(val) from tb where name = a.name) order by a.name
方法二：
select a.* from tb a where not exists(select 1 from tb where name = a.name and val > a.val)
方法三：
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
方法四：
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
方法五：
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val > a.val ) order by a.name
方法六:
select * from (select * from tb ORDER BY val desc) temp GROUP BY name ORDER BY val desc;
以上六种方法运行的结果均为如下所示：
| name | val | memo |
|---|---|---|
| a | 3 | a3 |
| b | 5 | b5 |
小编推荐使用第一、第三、第四钟方法,结果显示第1,3,4种方法效率相同，第2，5种方法效率差些。
按name分组取val最小的值所在行的数据
方法一：
select a.* from tb a where val = (select min(val) from tb where name = a.name) order by a.name
方法二：
select a.* from tb a where not exists(select 1 from tb where name = a.name and val < a.val)
方法三：
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
方法四：
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
方法五：
select a.* from tb a where 1 > (select count(*) from tb where name = a.name and val < a.val) order by a.name
以上五种方法运行的结果均为如下所示：
| name | val | memo |
|---|---|---|
| a | 1 | a1 |
| b | 1 | b1 |
按name分组取第一次出现的行所在的数据
sql如下：
select a.* from tb a where val = (select top 1 val from tb where name = a.name) order by a.name
//这个是sql server的
//mysql应该是
select a.* from tb a where val = (select val from tb where name = a.name limit 1) order by a.name
结果如下：
| name | val | memo |
|---|---|---|
| a | 2 | a2 |
| b | 1 | b1 |
-----下面的没有验证-- 感觉是sql-server的写法,mysql的随机是rand(),前几条记录是limit N.
按name分组随机取一条数据
sql如下：
select a.* from tb a where val = (select top 1 val from tb where name = a.name order by newid()) order by a.name
结果如下：
| name | val | memo |
|---|---|---|
| a | 1 | a1 |
| b | 3 | b3 |
按name分组取最小的两个(N个)val
第一种方法：
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
第二种方法：
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
第三种方法：
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
结果如下：
| name | val | memo |
|---|---|---|
| a | 1 | a1 |
| a | 2 | a2 |
| b | 1 | b1 |
| b | 2 | b2 |
按name分组取最大的两个(N个)val
第一种方法：
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
第二种方法：
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
第三种方法：
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
结果如下：
| name | val | memo |
|---|---|---|
| a | 3 | a3 |
| a | 2 | a2 |
| b | 5 | b5 |
| b | 4 | b4 |
转:http://www.manongjc.com/article/1082.html