mysql的group by语法可以根据指定的规则对数据进行分组，分组就是将一个数据集划分成若干个小区域，然后再针对若干个小区域进行数据处理。本文将介绍mysql使用group by分组时，实现组内排序的方法。
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mysql的group by语法可以对数据进行分组，但是分组后的数据并不能进行组内排序。
例如一个评论表有多个用户评论，需要获取每个用户最后评论的内容。
创建测试数据表及数据
CREATE TABLE `comment` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `user_id` int(10) unsigned NOT NULL, `content` varchar(200) NOT NULL, `addtime` datetime NOT NULL, `lastmodify` datetime NOT NULL, PRIMARY KEY (`id`), KEY `user_id` (`user_id`), KEY `addtime` (`addtime`), KEY `uid_addtime` (`user_id`,`addtime`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `comment` (`id`, `user_id`, `content`, `addtime`, `lastmodify`) VALUES(1, 1, '评论1', '2017-05-17 00:00:00', '2017-05-17 00:00:00'), (2, 1, '评论2', '2017-05-17 00:00:01', '2017-05-17 00:00:01'), (3, 2, '评论1', '2017-05-17 00:00:02', '2017-05-17 00:00:02'), (4, 2, '评论2', '2017-05-17 00:00:03', '2017-05-17 00:00:03'), (5, 3, '评论1', '2017-05-17 00:00:04', '2017-05-17 00:00:04'), (6, 1, '评论3', '2017-05-17 00:00:05', '2017-05-17 00:00:05'), (7, 4, '评论1', '2017-05-17 00:00:06', '2017-05-17 00:00:06'), (8, 4, '评论2', '2017-05-17 00:00:07', '2017-05-17 00:00:07'), (9, 4, '评论3', '2017-05-17 00:00:08', '2017-05-17 00:00:08'), (10, 4, '评论4', '2017-05-17 00:00:09', '2017-05-17 00:00:09'), (11, 3, '评论2', '2017-05-17 00:00:10', '2017-05-17 00:00:10');select * from comment;+----+---------+---------+---------------------+---------------------+| id | user_id | content | addtime | lastmodify | +----+---------+---------+---------------------+---------------------+| 1 | 1 | 评论1 | 2017-05-17 00:00:00 | 2017-05-17 00:00:00 | | 2 | 1 | 评论2 | 2017-05-17 00:00:01 | 2017-05-17 00:00:01 | | 3 | 2 | 评论1 | 2017-05-17 00:00:02 | 2017-05-17 00:00:02 | | 4 | 2 | 评论2 | 2017-05-17 00:00:03 | 2017-05-17 00:00:03 | | 5 | 3 | 评论1 | 2017-05-17 00:00:04 | 2017-05-17 00:00:04 | | 6 | 1 | 评论3 | 2017-05-17 00:00:05 | 2017-05-17 00:00:05 | | 7 | 4 | 评论1 | 2017-05-17 00:00:06 | 2017-05-17 00:00:06 | | 8 | 4 | 评论2 | 2017-05-17 00:00:07 | 2017-05-17 00:00:07 | | 9 | 4 | 评论3 | 2017-05-17 00:00:08 | 2017-05-17 00:00:08 | | 10 | 4 | 评论4 | 2017-05-17 00:00:09 | 2017-05-17 00:00:09 | | 11 | 3 | 评论2 | 2017-05-17 00:00:10 | 2017-05-17 00:00:10 | +----+---------+---------+---------------------+---------------------+
在comment表中，每个用户最后评论的内容就是id为6,4,11,10的记录。
使用group by查询
select * from comment group by user_id; +----+---------+---------+---------------------+---------------------+| id | user_id | content | addtime | lastmodify | +----+---------+---------+---------------------+---------------------+| 1 | 1 | 评论1 | 2017-05-17 00:00:00 | 2017-05-17 00:00:00 | | 3 | 2 | 评论1 | 2017-05-17 00:00:02 | 2017-05-17 00:00:02 | | 5 | 3 | 评论1 | 2017-05-17 00:00:04 | 2017-05-17 00:00:04 || 7 | 4 | 评论1 | 2017-05-17 00:00:06 | 2017-05-17 00:00:06 | +----+---------+---------+---------------------+---------------------+
可以看到结果，分组后只会返回分组内的第一条数据。因为group by语法没有进行组内排序的功能，只会按mysql默认的排序显示。
如何才能对group by分组内的数据进行排序了，这个需要根据不同的需求处理。
1.id最大的，评论时间肯定最新
这种情况我们可以使用id代替时间去搜寻并组内排序，使用max(id)就可以获取到每个分组中最大的评论id(即最新的评论)
select * from comment where id in(select max(id) from comment group by user_id) order by user_id; +----+---------+---------+---------------------+---------------------+| id | user_id | content | addtime | lastmodify | +----+---------+---------+---------------------+---------------------+| 6 | 1 | 评论3 | 2017-05-17 00:00:05 | 2017-05-17 00:00:05 | | 4 | 2 | 评论2 | 2017-05-17 00:00:03 | 2017-05-17 00:00:03 | | 11 | 3 | 评论2 | 2017-05-17 00:00:10 | 2017-05-17 00:00:10 || 10 | 4 | 评论4 | 2017-05-17 00:00:09 | 2017-05-17 00:00:09 | +----+---------+---------+---------------------+---------------------+
2.id与评论时间没有关系，id大的评论时间可能不是最新
这种情况我们就需要使用max(addtime)来获取最新的评论，但因为不同用户的评论时间有可能相同，因此还需要加多user_id这个条件去查询。
重新创建测试数据
truncate table comment;INSERT INTO `comment` (`id`, `user_id`, `content`, `addtime`, `lastmodify`) VALUES(1, 1, '评论1', '2017-05-17 00:00:00', '2017-05-17 00:00:00'), (2, 1, '评论2', '2017-05-17 00:10:01', '2017-05-17 00:10:01'), (3, 2, '评论1', '2017-05-17 00:10:02', '2017-05-17 00:10:02'), (4, 2, '评论2', '2017-05-17 00:00:03', '2017-05-17 00:00:03'), (5, 3, '评论1', '2017-05-17 00:10:04', '2017-05-17 00:10:04'), (6, 1, '评论3', '2017-05-17 00:00:05', '2017-05-17 00:00:05'), (7, 4, '评论1', '2017-05-17 00:00:06', '2017-05-17 00:00:06'), (8, 4, '评论2', '2017-05-17 00:10:07', '2017-05-17 00:10:07'), (9, 4, '评论3', '2017-05-17 00:00:08', '2017-05-17 00:00:08'), (10, 4, '评论4', '2017-05-17 00:00:09', '2017-05-17 00:00:09'), (11, 3, '评论2', '2017-05-17 00:00:10', '2017-05-17 00:00:10');select * from comment;+----+---------+---------+---------------------+---------------------+| id | user_id | content | addtime | lastmodify | +----+---------+---------+---------------------+---------------------+| 1 | 1 | 评论1 | 2017-05-17 00:00:00 | 2017-05-17 00:00:00 | | 2 | 1 | 评论2 | 2017-05-17 00:10:01 | 2017-05-17 00:10:01 | | 3 | 2 | 评论1 | 2017-05-17 00:10:02 | 2017-05-17 00:10:02 | | 4 | 2 | 评论2 | 2017-05-17 00:00:03 | 2017-05-17 00:00:03 | | 5 | 3 | 评论1 | 2017-05-17 00:10:04 | 2017-05-17 00:10:04 | | 6 | 1 | 评论3 | 2017-05-17 00:00:05 | 2017-05-17 00:00:05 | | 7 | 4 | 评论1 | 2017-05-17 00:00:06 | 2017-05-17 00:00:06 | | 8 | 4 | 评论2 | 2017-05-17 00:10:07 | 2017-05-17 00:10:07 | | 9 | 4 | 评论3 | 2017-05-17 00:00:08 | 2017-05-17 00:00:08 | | 10 | 4 | 评论4 | 2017-05-17 00:00:09 | 2017-05-17 00:00:09 | | 11 | 3 | 评论2 | 2017-05-17 00:00:10 | 2017-05-17 00:00:10 | +----+---------+---------+---------------------+---------------------+
符合条件的应该是id为2,3,5,8的记录
select a.* from comment as a right join (select user_id, max(addtime) as maxtime from comment where user_id is not null group by user_id) as b on a.user_id=b.user_id and a.addtime=b.maxtime order by a.user_id asc;+------+---------+---------+---------------------+---------------------+| id | user_id | content | addtime | lastmodify | +------+---------+---------+---------------------+---------------------+| 2 | 1 | 评论2 | 2017-05-17 00:10:01 | 2017-05-17 00:10:01 | | 3 | 2 | 评论1 | 2017-05-17 00:10:02 | 2017-05-17 00:10:02 | | 5 | 3 | 评论1 | 2017-05-17 00:10:04 | 2017-05-17 00:10:04 | | 8 | 4 | 评论2 | 2017-05-17 00:10:07 | 2017-05-17 00:10:07 | +------+---------+---------+---------------------+---------------------+
使用right join可以减少外层的数据集。
where user_id is not null 可以使group by user_id时使用索引。
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