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  • 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 3 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

    同样问题:http://www.cnblogs.com/fps2tao/p/9038268.html

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  • 原文地址:https://www.cnblogs.com/fps2tao/p/9041175.html
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