需求：
表中同一个uid（用户）拥有多条游戏等级记录，现需要取所有用户最高等级（level）的那一条数据，且时间（time）越早排越前。这是典型的排名表
+------+-------+--------------+---------------------+
| uid | level | role | time |
+------+-------+--------------+---------------------+
| 7 | 1 | 摇滚圣魔 | 2014-06-12 15:01:05 |
| 1134 | 4 | 唯我独尊 | 2014-06-12 15:02:38 |
| 1134 | 4 | 唯我独尊 | 2014-06-12 15:02:39 |
| 7 | 3 | 摇滚圣魔 | 2014-06-12 15:02:59 |
| 5 | 3 | 韵儿铃♦ | 2014-06-12 15:04:09 |
| 7363 | 6 | 诗荷冰月 | 2014-06-12 15:04:23 |
| 6684 | 4 | つ道远虚空つ | 2014-06-12 15:05:13 |
| 7 | 16 | 摇滚圣魔 | 2014-06-12 15:05:46 |
| 1 | 2 | 斗土豪 | 2014-06-12 15:05:48 |
| 7 | 26 | 摇滚圣魔 | 2014-06-12 15:08:36 |
| 6684 | 8 | つ道远虚空つ | 2014-06-12 15:08:45 |
| 5 | 12 | 韵儿铃♦ | 2014-06-12 15:09:47 |
| 6834 | 1 | 无敌追翼 | 2014-06-12 15:10:56 |
| 5 | 16 | 韵儿铃♦ | 2014-06-12 15:11:18 |
| 8719 | 1 | 君望赤 | 2014-06-12 15:11:48 |
| 6274 | 36 | 五月独孤 | 2014-06-12 15:12:22 |
| 8724 | 26 | 童童 | 2014-06-12 15:12:31 |
| 1134 | 32 | 唯我独尊 | 2014-06-12 15:12:51 |
| 7 | 26 | 摇滚圣魔 | 2014-06-12 15:13:38 |
| 5 | 25 | 韵儿铃♦ | 2014-06-12 15:14:48 |
| 7757 | 3 | つ清灵旋つ | 2014-06-12 15:16:50 |
| 7 | 26 | 摇滚圣魔 | 2014-06-12 15:17:26 |
| 5 | 28 | 韵儿铃♦ | 2014-06-12 15:18:08 |
| 7757 | 23 | つ清灵旋つ | 2014-06-12 15:19:29 |
| 6274 | 43 | 五月独孤 | 2014-06-12 15:19:54 |
| 8724 | 30 | 童童 | 2014-06-12 15:20:39 |
| 7757 | 26 | つ清灵旋つ | 2014-06-12 15:20:58 |
| 8707 | 36 | 就是干 | 2014-06-12 15:22:28 |
| 7757 | 29 | つ清灵旋つ | 2014-06-12 15:23:05 |
| 7757 | 32 | つ清灵旋つ | 2014-06-12 15:24:57 |
| 8726 | 10 | 连曦 | 2014-06-12 15:26:01 |
| 7363 | 34 | 诗荷冰月 | 2014-06-12 15:26:58 |
| 7 | 26 | 摇滚圣魔 | 2014-06-12 15:27:33 |
| 5 | 37 | 韵儿铃♦ | 2014-06-12 15:27:37 |
| 8347 | 1 | 无敌 | 2014-06-12 15:28:09 |
| 6274 | 47 | 五月独孤 | 2014-06-12 15:28:13 |
| 1 | 32 | 斗土豪 | 2014-06-12 15:29:18 |
| 1134 | 46 | 唯我独尊 | 2014-06-12 15:30:52 |
| 7757 | 41 | つ清灵旋つ | 2014-06-12 15:30:56 |
| 9 | 34 | 饭饭饭饭の | 2014-06-12 15:31:03 |
| 6274 | 48 | 五月独孤 | 2014-06-12 15:31:18 |
| 8724 | 41 | 童童 | 2014-06-12 15:32:30 |
| 7757 | 42 | つ清灵旋つ | 2014-06-12 15:34:24 |
| 1134 | 48 | 唯我独尊 | 2014-06-12 15:34:56 |
| 1100 | 2 | 圣魔霄 | 2014-06-12 15:35:54 |
| 1008 | 21 | ∵嘟嘟冰儿∵ | 2014-06-12 15:36:10 |
| 7757 | 45 | つ清灵旋つ | 2014-06-12 15:40:49 |
| 3088 | 4 | 战魂☼凌空 | 2014-06-12 15:41:38 |
| 5 | 41 | 韵儿铃♦ | 2014-06-12 15:41:56 |
| 7757 | 46 | つ清灵旋つ | 2014-06-12 15:43:24 |
实现过程：
1.首先对该数据进行2次排序，uid 排序以及等级的降序排序
2.对排序后的结果用uid进行分组,分组后等级降序排序,时间升序排序
实现SQL:
select * from (select a.uid,a.level,a.role,a.time from 数据表 a where order BY a.uid desc,a.level desc) as 别名 group by user_uid order by level desc,time asc limit 30;
