Mysql 数据分组取某字段值所有最大的记录行

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;