如下表：
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；
數據如下：
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)
其中userid不唯一，要求取表中每個userid對應的時間離現在最近的一條記錄．初看到一個這條件一般都會想到借用臨時表及添加主建借助於join操作之類的．
給一個簡方法：
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)
Good luck!
