title: MySQL Select rows on first occurrence of each unique value
body: <p>Let's say you have the following table (the column of interest here is <code>cid</code>):</p>
  
  <pre><code>+-----+-------+-------+-------+---------------------+--------------+
  | 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 |
  +-----+-------+-------+-------+---------------------+--------------+
  </code></pre>
  
  <p>How to extract only the first occurrence of each <code>cid</code>? The resulting table would be:</p>
  
  <pre><code>+-----+-------+-------+-------+---------------------+--------------+
  | 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 :)        |
  +-----+-------+-------+-------+---------------------+--------------+
  </code></pre>