MySQL在每个唯一值的首次出现时选择行 [英] MySQL Select rows on first occurrence of each unique value

问题描述

假设您有下表(这里感兴趣的列是cid):

Let's say you have the following table (the column of interest here is 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 |
+-----+-------+-------+-------+---------------------+--------------+

如何仅提取每个cid的第一个匹配项?结果表将是:

How to extract only the first occurrence of each cid? The resulting table would be:

+-----+-------+-------+-------+---------------------+--------------+
| 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 :)        |
+-----+-------+-------+-------+---------------------+--------------+

推荐答案

mysql为此有一个骗子":

mysql has a "cheat" for this:

select *
from mytable
group by cid;

这就是您所需要的，因为在mysql中，它允许您不聚集非分组列(其他数据库会引发语法错误)，在这种情况下，它仅输出第一次出现的列每个分组值的.请注意，尽管这不能保证确定首次"出现的方式(这只是读取行的方式)

That's all you need, because in mysql it allows you to not aggregate the non-grouped-by columns (other databases would throw a syntax error), in which case it outputs only the first occurrence of each group-by value(s). Note though that this won't guarantee the way in which the "first" occurrence is determined (it will be just how the rows are read in)

如果您想首先出现特殊，请先进行排序，然后应用分组欺骗:

If you want a particular first occurrence, sort first, then apply the group-by cheat:

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

这篇关于MySQL在每个唯一值的首次出现时选择行的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！