MySQL：删除除最后 N 条记录以外的所有记录

Question

考虑下表：

mysql> DESCRIBE pixels;
+---------------+-------------+------+-----+-------------------+----------------+
| Field         | Type        | Null | Key | Default           | Extra          |
+---------------+-------------+------+-----+-------------------+----------------+
| id            | bigint(20)  | NO   | PRI | NULL              | auto_increment |
| pixel_id      | varchar(32) | NO   | MUL | NULL              |                |
| creation_time | timestamp   | NO   | MUL | CURRENT_TIMESTAMP |                |
| pixel         | mediumblob  | NO   |     | NULL              |                |
+---------------+-------------+------+-----+-------------------+----------------+
4 rows in set (0.04 sec)

用一些数据：

mysql> SELECT * FROM pixels;
+----+----------------------------------+---------------------+----------------------------------+
| id | pixel_id                         | creation_time       | pixel                            |
+----+----------------------------------+---------------------+----------------------------------+
|  1 | 0d1b042671e0f8c1d1f226abe923583c | 2012-07-01 14:42:26 | 2d8292a62e89fcbf8b1592cf53f0dc86 |
|  2 | 9192b7491ac9321ed67c198834965580 | 2012-07-01 14:42:26 | f41a4a3e1a5f2f25c02f2e377627355c |
|  3 | 82b6ad645a4c75a552c0ddfd8d07c38a | 2012-07-01 14:42:27 | bffd2be16fcb82d0592aaa00fe0ebb9d |
|  4 | de41f4932ee7e90bed2e26d4e7e1937a | 2012-07-01 14:42:27 | 6632df3642ce3465ee5160126f20d837 |
|  5 | f98ac2c09574e2accb6cff709ac8a97f | 2012-07-01 14:42:27 | 00d1a3d9e9b51d7e5f66120203189107 |
|  6 | e90a3233fd9054fb3c23d04b03a8dde8 | 2012-07-01 14:42:27 | 4d20a996a46b9767d8c3f6708cb0ce88 |
|  7 | 08177f9f44f3d6fa515bd1a1983a7b45 | 2012-07-01 14:42:28 | ed3a572da6d05d34f5928035bc67d5be |
|  8 | 9d9138ffb7df537d61276a91e837a327 | 2012-07-01 14:42:28 | 3be6876351254ffa4a00364cd3e8c10e |
+----+----------------------------------+---------------------+----------------------------------+

我正在尝试删除除最后 5 行之外的所有记录。“最后”由 确定id，creation_time可能重复。

我试过了：

mysql> SELECT id FROM pixels ORDER BY creation_time DESC LIMIT 1 OFFSET 5;

并得到了合理的答案：

+----+
| id |
+----+
|  3 |
+----+
1 row in set (0.00 sec)

我尝试将其用作子查询：

mysql> DELETE FROM pixels WHERE id < 
      (SELECT id FROM pixels ORDER BY creation_time DESC LIMIT 1 OFFSET 5);
ERROR 1093 (HY000): You can't specify target table 'pixels' for update in FROM clause

尝试了另一种方法：

mysql> DELETE FROM pixels WHERE id NOT IN 
      (SELECT id FROM pixels ORDER BY creation_time DESC LIMIT 5);
ERROR 1235 (42000): This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'

那么，如何删除除 N 个最近的记录之外的所有记录？

Answer 1

您需要稍微更改您的子查询方法 - 将条件从WHERE子句移动到连接，以绕过 MySQL 限制。

如果id和creation_time始终定义相同的顺序，则可以使用以下命令：

DELETE p
FROM 
       pixels AS p
   JOIN
       ( SELECT id 
         FROM pixels 
         ORDER BY id       
           LIMIT 1 OFFSET 4
       ) AS lim
     ON p.id < lim.id ;

由于这可能不是真的，并且两个排序有时可能不同，导致删除的行数比 5 多或少，您可以使用它，这会更准确，但对于大表可能会更慢：

DELETE p
FROM 
       pixels AS p
   JOIN
       ( SELECT creation_time, id 
         FROM pixels 
         ORDER BY creation_time DESC, id DESC
           LIMIT 1 OFFSET 4
       ) AS lim
     ON p.creation_time < lim.creation_time
     OR p.creation_time = lim.creation_time AND p.id < lim.id ;