MySQL 【去重留一】一条sql语句完成思路总结

太过爱你忘了你带给我的痛 2022-02-01 11:04 231阅读 0赞

# MySQL 【去重留一】一条sql语句完成 思路总结  #

[![MySQL.jpg][]][MySQL.jpg]

[MySQL.jpg][]

前几天在做一个需求的时候，需要清理mysql中重复的记录，当时的想法是通过代码遍历写出来，然后觉得太复杂，心里想着应该可以通过一个sql语句来解决问题的。查了资料，请教了大佬之后得出了一个很便利的sql语句，这里分享下这段sql语句和思路。

Nothing on you - 邓福如

Beautiful girls all over the world. (世界上有那么多的漂亮女孩)

I could be chasing but my time will be wasted. (我可以去追她们 不过那只会浪费我的时间)

They got nothin' on you baby. (她们都比不上你,宝贝)

Nothin' on you baby. (没人比得上你,宝贝)

They might say hi and I might say hey. (她们会说嗨我也会说嘿)

But you shouldn't worry about what they say. (但是你不用担心她们说的)

Coz they got nothin' on you baby. (因为她们哪儿及得上你，宝贝)

Nothin' on you baby. (及不上你，宝贝)

I know you feel where I'm coming from. (我知道你能看得出 我是什么样的人)

So baby regardless of the things in my past everything I've done. (不管过去我做过什么荒唐事)

Cause you and I know it was just for fun, I was on the carrousel I just spun around. (大多数都是玩玩罢了 就像旋转木马般,我已晕头转向)

With no direction, no intentions. It all ended. (没有任何方向,只是试图追求什么)

It's so much nonsense, it's on my conscience (做过太多愚蠢的事 我扪心自问)

I'm thinking "maybe I should get it out" (我在想我应该将它抛开)

And I don't wanna sound redundant (我不想让它听起来像是一句废话)

But I was wondering, if there was something that you wanna know (不过我曾想过有没有哪些事是你想知道的)

But never mind that, we should let it go (不过那都不重要 把它给忘了吧)

Coz we don't wanna be a T.V. episode (因为我们不想搞得像在演连续剧)

And all the bad thoughts, just let em go, go, go (所有的烦恼,就让它们全部溜走)

Beautiful girls all over the world. (世界上有那么多的漂亮女孩)

I could be chasing but my time will be wasted. (我可以去追她们 不过那只会浪费我的时间)

They got nothin' on you baby. (她们都比不上你,宝贝)

Nothin' on you baby. (没人比得上你,宝贝)

They might say hi and I might say hey. (她们会说嗨我也会说嘿)

But you shouldn't worry about what they say. (但是你不用担心她们说的)

Coz they got nothin' on you baby. (因为她们哪儿及得上你，宝贝)

Nothin' on you baby. (及不上你，宝贝)

I know you feel where I'm coming from. (我知道你能看得出 我是什么样的人)

So baby regardless of the things in my past everything I've done. (不管过去我做过什么荒唐事)

Cause you and I know it was just for fun, I was on the carrousel I just spun around. (大多数都是玩玩罢了 就像旋转木马般,我已晕头转向)

With no direction, no intentions. It all ended. (没有任何方向,只是试图追求什么)

It's so much nonsense, it's on my conscience (做过太多愚蠢的事 我扪心自问)

I'm thinking "maybe I should get it out" (我在想我应该将它抛开)

And I don't wanna sound redundant (我不想让它听起来像是一句废话)

But I was wondering, if there was something that you wanna know (不过我曾想过有没有哪些事是你想知道的)

But never mind that, we should let it go (不过那都不重要 把它给忘了吧)

Coz we don't wanna be a T.V. episode (因为我们不想搞得像在演连续剧)

And all the bad thoughts, just let em go, go, go (所有的烦恼,就让它们全部溜走)

Beautiful girls all over the world. (世界上有那么多的漂亮女孩)

I could be chasing but my time will be wasted. (我可以去追她们 不过那只会浪费我的时间)

They got nothin' on you baby. (她们都比不上你,宝贝)

Nothin' on you baby. (没人比得上你,宝贝)

They might say hi and I might say hey. (她们会说嗨我也会说嘿)

But you shouldn't worry about what they say. (但是你不用担心她们说的)

Coz they got nothin' on you baby. (因为她们哪儿及得上你，宝贝)

Nothin' on you baby. (及不上你，宝贝)

Beautiful girls all over the world. (世界上有那么多的漂亮女孩)

I could be chasing but my time will be wasted. (我可以去追她们 不过那只会浪费我的时间)

They got nothin' on you baby. (她们都比不上你,宝贝)

Nothin' on you baby. (没人比得上你,宝贝)

They might say hi and I might say hey. (她们会说嗨我也会说嘿)

But you shouldn't worry about what they say. (但是你不用担心她们说的)

Coz they got nothin' on you baby. (因为她们哪儿及得上你，宝贝)

Nothin' on you baby. (及不上你，宝贝)

\- 00:00 / 04:04

## 需求分析 ##

> 数据库中存在重复记录，删除保留其中一条（是否重复判断基准为多个字段）

## 解决方案 ##

碰到这个需求的时候，心里大概是有思路的。最快想到的是可以通过一条sql语句来解决，无奈自己对于复杂sql语句的道行太浅，所以想找大佬帮忙。

### 找人帮忙 ###

因为这个需求有点着急，所以最开始想到的是，可以找这方面的同行来解决，然后分享这个问题给[@赵七七][Link 1]同学，结果这货随便百度了一下，就甩给我一个从未用过的sql语句，让我自己尝试，心里万匹那啥啥啥奔腾而过...

### 自己百度 ###

找到了一条sql语句：

DELETE
    FROM
        vitae a
    WHERE
        (a.peopleId, a.seq) IN (
            SELECT
                peopleId,
                seq
            FROM
                vitae
            GROUP BY
                peopleId,
                seq
            HAVING
                count(*) > 1
        )
    AND rowid NOT IN (
        SELECT
            min(rowid)
        FROM
            vitae
        GROUP BY
            peopleId,
            seq
        HAVING
            count(*) > 1
    )

这条语句是在[【MySQL中删除重复数据只保留一条】][MySQL]这篇文章里找到的。这条sql思路很明显，有以下3步：

1.  `SELECT peopleId, seq FROM vitae GROUP BY peopleId, seq HAVING count(*) > 1`查询出表中重复记录作为条件
2.  `SELECT min(rowid) FROM vitae GROUP BY peopleId, seq HAVING count(*) > 1`查询出表中重复记录中ID最小的值为第二个条件
3.  最后根据以上两个条件，删除**除**重复记录中最小ID的其余重复记录

但是很无奈的是，运行这条语句出现了错误，大致报错意思是，不能在查询的时候同时更新这个表。

### 代码解决 ###

根据上面这个sql语句想到或许可以通过代码的方式，两步来达到同样的目的：

1.  先取出重复的数据集
2.  根据查询到的数据集，循环删除其余的重复数据

想法是有了，写出来也很快，但是一运行吓我一跳，竟然需要**116s**左右，然后自己就想一定要找到可以使用的sql语句，贴一下代码和运行结果：

[![code.png][]][code.png]

[code.png][]

[![run\_code\_result.png][run_code_result.png]][run_code_result.png]

[run\_code\_result.png][run_code_result.png]

### 完美的【去重留一】SQL ###

最后在一个技术群里得到了完美的答案，看这条sql语句：

DELETE consum_record
    FROM
        consum_record, 
        (
            SELECT
                min(id) id,
                user_id,
                monetary,
                consume_time
            FROM
                consum_record
            GROUP BY
                user_id,
                monetary,
                consume_time
            HAVING
                count(*) > 1
        ) t2
    WHERE
        consum_record.user_id = t2.user_id 
        and consum_record.monetary = t2.monetary
        and consum_record.consume_time  = t2.consume_time
    AND consum_record.id > t2.id;

上面这条sql语句，仔细看一下，揣摩出思路也不难，大概也分为3步来理解：

1.  `(SELECT min(id) id, user_id, monetary, consume_time FROM consum_record GROUP BY user_id, monetary, consume_time HAVING count(*) > 1 ) t2` 查询出重复记录形成一个集合（临时表t2），集合里是每种重复记录的最小ID
2.  `consum_record.user_id = t2.user_id and consum_record.monetary = t2.monetary and consum_record.consume_time = t2.consume_time` **关联**判断重复基准的字段
3.  根据条件，删除原表中id大于t2中id的记录

看到这个语句的时候，心里想这也太厉害了。这么一个简单的sql语句，竟然可以解决这么复杂的问题，涨姿势了~  
运行起来也超级快，原先的代码循环执行，需要**116s**左右，而这里**0.3s**就可以了，厉害了~

[![perfect\_sql.png][perfect_sql.png]][perfect_sql.png]

[perfect\_sql.png][perfect_sql.png]

## 总结 ##

作为一个java程序猿，按理来说sql这里是不能拖后腿的，无奈实际中，需要照顾的事情太多，现在的sql水平也只是处于在一个普通的层次中，以后找机会一定要补一下这方面的知识。

## 资源 ##

今天就分享到这里啦。

[MySQL.jpg]: /images/20220201/8809cba9d60341ecae705608c82a9e4a.png
[Link 1]: http://zqiqi.cn/
[MySQL]: http://blog.csdn.net/u014723529/article/details/51218638
[code.png]: /images/20220201/ec9eaf03593d47889406f33f12ab47bf.png
[run_code_result.png]: /images/20220201/681c387a0fac4b97b0a5b86f51546de9.png
[perfect_sql.png]: /images/20220201/35eb0e16b9564459a8dc363f0bca89ae.png