假设您有下表（这里关注的列是cid）：
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 :) | +-----+-------+-------+-------+---------------------+--------------+
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
