MySQL在每个唯一值的首次出现时选择行

Question

假设您有下表（这里关注的列是cid）：

+-----+-------+-------+-------+---------------------+--------------+
| cid | pid   | rid   | clink | time                | snippet      |
+-----+-------+-------+-------+---------------------+--------------+
| 155 | 11222 |  1499 |  1137 | 2012-08-22 03:05:06 | hi           |
| 138 | 11222 |   241 |  1136 | 2012-08-21 05:25:00 | again        |
| 138 | 11222 |   241 |  1135 | 2012-08-21 05:16:40 | hi           |
| 155 | 11222 |  1499 |  1134 | 2012-08-21 05:11:00 | hi cute      |
| 140 | 11222 | 11223 |  1133 | 2012-08-21 05:05:18 | hi           |
| 154 | 11222 |   565 |  1132 | 2012-08-21 05:04:47 | 7            |
| 153 | 11222 |   272 |  1131 | 2012-08-21 05:04:41 | 6            |
| 146 | 11222 |   362 |  1130 | 2012-08-21 05:04:33 | 5            |
| 152 | 11222 |   364 |  1129 | 2012-08-21 05:04:27 | 4            |
| 151 | 11222 |   390 |  1128 | 2012-08-21 05:04:22 | 3            |
| 150 | 11222 |   333 |  1127 | 2012-08-21 05:04:16 | 2            |
| 148 | 11222 |   268 |  1126 | 2012-08-21 05:04:10 | 1            |
| 140 | 11222 | 11223 |  1125 | 2012-08-21 04:59:57 | hi sir       |
| 147 | 11222 |   283 |  1123 | 2012-08-21 03:29:55 | yo           |
| 140 | 11222 | 11223 |  1121 | 2012-08-21 02:12:13 | hello!       |
| 139 | 11222 |   249 |  1120 | 2012-08-21 02:11:53 | hi :)        |
| 140 | 11222 | 11223 |  1119 | 2012-08-21 02:11:26 | hi :)        |
| 140 | 11222 | 11223 |  1118 | 2012-08-21 02:11:08 | hi too       |
| 139 | 11222 |   249 |  1117 | 2012-08-21 02:11:00 | :P           |
| 139 | 11222 |   249 |  1116 | 2012-08-21 02:10:57 | hi           |
| 139 | 11222 |   249 |  1115 | 2012-08-21 02:10:51 | helo         |
| 139 | 11222 |   249 |  1114 | 2012-08-21 02:06:19 | hi           |
| 139 | 11222 |   249 |  1113 | 2012-08-21 02:05:45 | hi baby      |
| 139 | 11222 |   249 |  1112 | 2012-08-21 02:05:00 | hi           |
| 139 | 11222 |   249 |  1111 | 2012-08-21 02:04:41 | hi           |
| 140 | 11222 | 11223 |  1110 | 2012-08-21 02:04:26 | hi           |
| 140 | 11222 | 11223 |  1108 | 2012-08-21 01:47:40 | hey :)       |
| 139 | 11222 |   249 |  1107 | 2012-08-21 01:44:43 | hi           |
| 138 | 11222 |   241 |  1106 | 2012-08-21 01:44:11 | hi           |
| 138 | 11222 |   241 |  1105 | 2012-08-21 01:09:20 | conv 1 msg 1 |
+-----+-------+-------+-------+---------------------+--------------+

如何只提取每个的第一次出现cid？结果表将是：

+-----+-------+-------+-------+---------------------+--------------+
| cid | pid   | rid   | clink | time                | snippet      |
+-----+-------+-------+-------+---------------------+--------------+
| 155 | 11222 |  1499 |  1137 | 2012-08-22 03:05:06 | hi           |
| 138 | 11222 |   241 |  1136 | 2012-08-21 05:25:00 | again        |
| 140 | 11222 | 11223 |  1133 | 2012-08-21 05:05:18 | hi           |
| 154 | 11222 |   565 |  1132 | 2012-08-21 05:04:47 | 7            |
| 153 | 11222 |   272 |  1131 | 2012-08-21 05:04:41 | 6            |
| 146 | 11222 |   362 |  1130 | 2012-08-21 05:04:33 | 5            |
| 152 | 11222 |   364 |  1129 | 2012-08-21 05:04:27 | 4            |
| 151 | 11222 |   390 |  1128 | 2012-08-21 05:04:22 | 3            |
| 150 | 11222 |   333 |  1127 | 2012-08-21 05:04:16 | 2            |
| 148 | 11222 |   268 |  1126 | 2012-08-21 05:04:10 | 1            |
| 147 | 11222 |   283 |  1123 | 2012-08-21 03:29:55 | yo           |
| 140 | 11222 | 11223 |  1121 | 2012-08-21 02:12:13 | hello!       |
| 139 | 11222 |   249 |  1120 | 2012-08-21 02:11:53 | hi :)        |
+-----+-------+-------+-------+---------------------+--------------+

Answer 1

mysql对此有一个“作弊”：

select *
from mytable
group by cid;

这就是你所需要的，因为在MySQL中它可以让你 没有
聚集不分组，按列（其他数据库将抛出一个语法错误），在这种情况下，输出仅对每个第一次出现组由值（S）。不过，请注意，这将无法保证的 方式
中，“第一”发生确定（这将是行只是如何读）

如果您要先出现某个 特定的东西， 请先进行排序，然后再应用分组秘籍：

select *
from (
    -- order by the "time" column descending to get the "most recent" row
    select * from mytable order by time desc
    ) x
group by cid