通过限制和条件在MySQL中删除记录

我在MySQL数据库中有一个名为log的表。它用于记录用户操作。当它变得非常大时，我想删除一些记录。通过限制和条件在MySQL中删除记录

SELECT `userid`, `timestamp` 
FROM `log` 
ORDER `timestamp` ASC

输出

userid timestamp 
    2 120000 
    3 123333 
    1 123456 
    1 124444 
    2 125555 
    2 126666 
    1 127777 
    1 128888 
    2 129999 
    3 130000 
    1 131111

我想要做的是 - 我想只保留最后3条为每个用户。因此，我需要删除每个用户的第4，第5，...，第n个记录。根据上面的示例，希望输出

userid timestamp 
    3 123333 
    2 125555 
    2 126666 
    1 127777 
    1 128888 
    2 129999 
    3 130000 
    1 131111

我知道，记录可以通过使用LIMIT被删除。

DELETE FROM `log` LIMIT 3

只删除3条记录。它根本无法得到我的愿望结果。

我已经试过是

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

而且这还不是我想要的。

来源

2014-01-08 Tun Zarni Kyaw

试试这个：

DELETE l FROM `log` l 
LEFT JOIN (SELECT l.userid, l.timestamp 
      FROM (SELECT l.userid, l.timestamp, 
         IF(@lastUserId = @lastUserId:=userid, @Idx:[email protected]+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

编辑：

DELETE l FROM `log` l 
WHERE NOT EXISTS (
      SELECT 1 
      FROM (SELECT l.userid, l.timestamp, 
         IF(@lastUserId = @lastUserId:=userid, @Idx:[email protected]+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 
     )

检查SQL FIDDLE DEMO

输出

| USERID | TIMESTAMP | 
|--------|-----------| 
|  3 | 123333 | 
|  2 | 125555 | 
|  2 | 126666 | 
|  1 | 127777 | 
|  1 | 128888 | 
|  2 | 129999 | 
|  3 | 130000 | 
|  1 | 131111 |

来源

2014-01-08 11:10:48

我得到的错误'＃1064'。我从答案中得到更多的想法。不管怎么说，还是要谢谢你。 –

@TunZarniKyaw检查我更新的答案 –

仍然得到错误＃1064，但是当我尝试...'SELECT * FROM（ SELECT l.userid，l.timestamp，IF（@lastUserId = @lastUserId：= userid，@Idx ：= @Idx 1，@Idx：= 0）ROWNUMBER FROM日志升，（ SELECT @lastUserId：= 0，@Idx：= 0 ）甲 ORDER BY l.userid，l.timestamp DESC ）甲 '是好的。它正确显示'rowNumber'。 –

试试下面的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

来源

2014-01-09 11:01:33 kwelsan

通过限制和条件在MySQL中删除记录

回答

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