mysql - 删除除最后三个记录之外的记录

Question

我有一个log 表，我想在其中删除除最后三个记录之外的每个用户的记录。

架构

DROP TABLE IF EXISTS `log`;
CREATE TABLE `log` (
  `user_id` int(11) DEFAULT NULL,
  `timestamp` int(11) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

insert  into `log`(`user_id`,`timestamp`) values (1,1389257013),(1,1389257014),(1,1389257015),(1,1389257016),(1,1389257017),(2,1389257018),(2,1389257019),(2,1389257020),(2,1389257021),(2,1389257022),(3,1389257023),(3,1389257024);

当前表:

id    timestamp
1     1389257013
1     1389257014
1     1389257015
1     1389257016
1     1389257017
2     1389257018
2     1389257019
2     1389257020
2     1389257021
2     1389257022
3     1389257023
3     1389257024

预期表

id    timestamp    
1     1389257015
1     1389257016
1     1389257017
2     1389257020
2     1389257021
2     1389257022
3     1389257023
3     1389257024

Answer 1

试试这个:

DELETE l FROM `log` l 
WHERE NOT EXISTS (
          SELECT 1
          FROM (SELECT l.user_id, l.timestamp, 
                       IF(@lastUserId = @lastUserId:=user_id, @Idx:=@Idx+1, @Idx:=0) rowNumber 
                FROM `log` l, (SELECT @lastUserId:=0, @Idx:=0) A
                ORDER BY l.user_id, l.timestamp DESC
               ) AS A
          WHERE l.user_id= A.user_idAND l.timestamp = A.timestamp AND rowNumber < 3
         );

检查 SQL FIDDLE DEMO

输出

| USER_ID |  TIMESTAMP |
|---------|------------|
|       1 | 1389257015 |
|       1 | 1389257016 |
|       1 | 1389257017 |
|       2 | 1389257020 |
|       2 | 1389257021 |
|       2 | 1389257022 |
|       3 | 1389257023 |
|       3 | 1389257024 |