在數據庫開發過程中，我們要為每種類型的數據取出前幾條記錄，或者是取最新、最小、最大等等，這個該如何實現呢，本文章向大家介紹如何實現mysql分組取最大(最小、最新、前N條)條記錄。需要的可以參考一下。
先看一下本示例中需要使用到的數據
創建表並插入數據：
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');
數據表如下：
| name | val | memo |
| a | 2 | a2 |
| a | 1 | a1 |
| a | 3 | a3 |
| b | 1 | b1 |
| b | 3 | b3 |
| b | 2 | b2 |
| b | 4 | b4 |
| b | 5 | b5 |
按name分組取val最大的值所在行的數據
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;
以上六種方法運行的結果均為如下所示：
| name | val | memo |
| a | 3 | a3 |
| b | 5 | b5 |
小編推薦使用第一、第三、第四鍾方法,結果顯示第1,3,4種方法效率相同，第2，5種方法效率差些。
按name分組取val最小的值所在行的數據
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
以上五種方法運行的結果均為如下所示：
| name | val | memo |
| a | 1 | a1 |
| b | 1 | b1 |
按name分組取第一次出現的行所在的數據
sql如下：
select a.* from tb a where val = (select top 1 val from tb where name = a.name) order by a.name //這個是sql server的 //mysql應該是 select a.* from tb a where val = (select val from tb where name = a.name limit 1) order by a.name
結果如下：
| name | val | memo |
| a | 2 | a2 |
| b | 1 | b1 |
-----下面的沒有驗證-- 感覺是sql-server的寫法,mysql的隨機是rand(),前幾條記錄是limit N.
按name分組隨機取一條數據
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結果如下：
| name | val | memo |
| a | 1 | a1 |
| b | 3 | b3 |
按name分組取最小的兩個(N個)val
第一種方法：
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第二種方法：
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第三種方法：
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結果如下：
| name | val | memo |
| a | 1 | a1 |
| a | 2 | a2 |
| b | 1 | b1 |
| b | 2 | b2 |
按name分組取最大的兩個(N個)val
第一種方法：
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第二種方法：
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第三種方法：
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結果如下：
| name | val | memo |
| a | 3 | a3 |
| a | 2 | a2 |
| b | 5 | b5 |
| b | 4 | b4 |
轉:http://www.manongjc.com/article/1082.html
同樣問題:http://www.cnblogs.com/fps2tao/p/9038268.html