[英]Delete records by limit and condition in MySQL
I have a table named log
in one of MySQL database. 我在MySQL数据库之一中有一个名为log
的表。 And it is used to log the user actions. 它用于记录用户操作。 When it becomes very large in size, I want to delete some records. 当它变得很大时,我想删除一些记录。
SELECT `userid`, `timestamp`
FROM `log`
ORDER `timestamp` ASC
the output is 输出是
userid timestamp
2 120000
3 123333
1 123456
1 124444
2 125555
2 126666
1 127777
1 128888
2 129999
3 130000
1 131111
What I want to do is that - I want to keep only last 3 records for each user. 我想做的是-我只想为每个用户保留最后3条记录 。 So, I need to delete the 4th, 5th, ..., nth records for each user. 因此,我需要为每个用户删除第4、5,...,n个记录。 According to the above sample, the desire output is 根据以上示例,期望输出为
userid timestamp
3 123333
2 125555
2 126666
1 127777
1 128888
2 129999
3 130000
1 131111
I know that records can be deleted by using LIMIT
. 我知道可以使用LIMIT
删除记录。
DELETE FROM `log` LIMIT 3
deletes only 3 records. 仅删除3条记录。 It simply cannot get my desire result. 它根本无法得到我想要的结果。
What I have tried is that 我试过的是
DELETE FROM
`log`
WHERE `userid` IN (
SELECT `userid` FROM (SELECT `userid`, COUNT(1) AS C
FROM `log`
GROUP BY `userid`
HAVING C > 3) CountTable ) LIMIT 3
And it is not what I wanted. 这不是我想要的。
Try this: 尝试这个:
DELETE l FROM `log` l
LEFT JOIN (SELECT l.userid, l.timestamp
FROM (SELECT l.userid, l.timestamp,
IF(@lastUserId = @lastUserId:=userid, @Idx:=@Idx+1, @Idx:=0) rowNumber
FROM `log` l, (SELECT @lastUserId:=0, @Idx:=0) A
ORDER BY l.userid, l.timestamp DESC
) AS A
WHERE rowNumber < 3
) AS A ON l.userid = A.userid AND l.timestamp = A.timestamp
WHERE A.userid IS NULL
EDIT: 编辑:
DELETE l FROM `log` l
WHERE NOT EXISTS (
SELECT 1
FROM (SELECT l.userid, l.timestamp,
IF(@lastUserId = @lastUserId:=userid, @Idx:=@Idx+1, @Idx:=0) rowNumber
FROM `log` l, (SELECT @lastUserId:=0, @Idx:=0) A
ORDER BY l.userid, l.timestamp DESC
) AS A
WHERE l.userid = A.userid AND l.timestamp = A.timestamp AND rowNumber < 3
)
Check the SQL FIDDLE DEMO 检查SQL FIDDLE DEMO
OUTPUT OUTPUT
| USERID | TIMESTAMP |
|--------|-----------|
| 3 | 123333 |
| 2 | 125555 |
| 2 | 126666 |
| 1 | 127777 |
| 1 | 128888 |
| 2 | 129999 |
| 3 | 130000 |
| 1 | 131111 |
Try below SQL: 请尝试以下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
)
)
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.