CREATE TABLE `t1` (`userid` int(11) DEFAULT NULL,`atime` datetime DEFAULT NULL,KEY `idx_userid` (`userid`)) ENGINE=InnoDB DEFAULT CHARSET=utf8； CREATE TABLE `t1` (`userid` int(11) DEFAULT NULL,`atime` datetime DEFAULT NULL,KEY `idx_userid` (`userid`)) ENGINE=InnoDB DEFAULT CHARSET=utf8；

MySQL> select * from t1;+--------+---------------------+| userid | atime |+--------+---------------------+| 1 | 2013-08-12 11:05:25 || 2 | 2013-08-12 11:05:29 || 3 | 2013-08-12 11:05:32 || 5 | 2013-08-12 11:05:34 || 1 | 2013-08-12 11:05:40 || 2 | 2013-08-12 11:05:43 || 3 | 2013-08-12 11:05:48 || 5 | 2013-08-12 11:06:03 |+--------+---------------------+8 rows in set (0.00 sec) MySQL> select * from t1;+--------+---------------------+| userid | atime |+--------+---------------------+| 1 | 2013-08-12 11:05:25 || 2 | 2013-08-12 11:05:29 || 3 | 2013-08-12 11:05:32 || 5 | 2013-08-12 11:05:34 || 1 | 2013-08-12 11:05:40 || 2 | 2013-08-12 11:05:43 || 3 | 2013-08-12 11:05:48 || 5 | 2013-08-12 11:06:03 |+--------+---------------------+8 rows in set (0.00 sec)

MySQL> select userid,substring_index(group_concat(atime order by atime desc),",",1) as atime from t1 group by userid;+--------+---------------------+| userid | atime |+--------+---------------------+| 1 | 2013-08-12 11:05:40 || 2 | 2013-08-12 11:05:43 || 3 | 2013-08-12 11:05:48 || 5 | 2013-08-12 11:06:03 |+--------+---------------------+4 rows in set (0.03 sec) MySQL> select userid,substring_index(group_concat(atime order by atime desc),",",1) as atime from t1 group by userid;+--------+---------------------+| userid | atime |+--------+---------------------+| 1 | 2013-08-12 11:05:40 || 2 | 2013-08-12 11:05:43 || 3 | 2013-08-12 11:05:48 || 5 | 2013-08-12 11:06:03 |+--------+---------------------+4 rows in set (0.03 sec)