MySQL在每个唯一值的第一次出现时选择行

Question

假设您有下表(这里感兴趣的列是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中它允许你不聚合非分组列(其他数据库会引发语法错误),在这种情况下它只输出每个group-by值的第一次出现.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)

如果您想要特定的第一次出现,请先排序,然后应用分组作弊:

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

Answer 2

试试这个，

SELECT *
FROM tableName a 
INNER JOIN (
    SELECT cid, MIN(`time`) AS MinTime
    FROM tableName
    GROUP BY cid
) b 
ON a.CID = B.cid AND a.time = b.MinTime

Answer 3

在MySQL 8，你可以使用窗口功能，此

SELECT cid, pid, rid, clink, time, snippet
FROM (
  SELECT t.*, ROW_NUMBER() OVER (PARTITION BY cid ORDER BY time) rn
  FROM t
) t
WHERE rn = 1