Delete Records Except Last Three Records

Question

I have a log table in which I want to delete records of each user except last three records.

Schema

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);

Current Table:

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

Expected Table

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

Answer 1

Try below SQL:

DELETE FROM log WHERE find_in_set(
    TIMESTAMP, (
        SELECT group_concat(t3) t4 FROM (
            SELECT 1 AS dummy,
            replace(group_concat(TIMESTAMP ORDER BY TIMESTAMP DESC), concat(SUBSTRING_INDEX(group_concat(TIMESTAMP ORDER BY TIMESTAMP DESC), ',', 3), ','), '') t3
            FROM log
            GROUP BY user_id HAVING count(*) > 3
        ) a GROUP BY dummy
    )
)

SQL Fiddle