来自数据库的最新数据 [英] Distinct most recent data from database

问题描述

我正在将数据存储在数据库中.存储的数据看起来像这样

I am storing data in my database. The data being stored looks like this

id  | upload_month | created_at
-----------------------------------------
1   | January      | 2017-01-30 13:22:39
-----------------------------------------
2   | Febuary      | 2017-01-30 13:23:42
-----------------------------------------
3   | January      | 2017-01-30 13:25:33

在我的控制器内，我尝试检索不同的upload_month，但获取每个文件的最新插入版本.此刻我正在尝试

Within my Controller I am trying to retrieve the distinct upload_month, but get the latest inserted version for each. At the moment I am trying

$uploadedFile = UploadedFile::groupBy('upload_month')->orderBy('created_at', 'desc')->get();

问题在于，这将返回以下内容

The problem is that this is returning the following

id  | upload_month | created_at
-----------------------------------------
1   | January      | 2017-01-30 13:22:39
-----------------------------------------
2   | Febuary      | 2017-01-30 13:23:42
-----------------------------------------

因此对于一月份的记录，它提供的是较旧的版本.如果将其更改为->orderBy('created_at', 'asc')，它将返回相同的记录，但Febuary是第一行.

So for the January record it is giving the older version. If I change it to ->orderBy('created_at', 'asc') it returns the same records but Febuary being the first row.

从本质上讲，我所追求的是这个

In essense, what I am after is this

id  | upload_month | created_at
-----------------------------------------
1   | January      | 2017-01-30 13:25:33
-----------------------------------------
2   | Febuary      | 2017-01-30 13:23:42
-----------------------------------------

我怎么能做到这一点?

谢谢

推荐答案

您应该GROUP BY您要选择的所有字段，而不仅仅是一个.本文介绍了该问题:

You should GROUP BY all fields you want to select, no only one. This article explain the issue: https://www.psce.com/blog/2012/05/15/mysql-mistakes-do-you-use-group-by-correctly/

在这种情况下，正确的SQL查询将是:

The proper SQL query in this case would be:

SELECT id, upload_month, created_at
  FROM uplodaded_file
  JOIN (SELECT upload_month, MAX(created_at) created_at
          FROM uplodaded_file
      GROUP BY upload_month) months
    ON upload_month = months.upload_month
   AND created_at = months.created_at

雄辩的版本有点棘手.在这种情况下，最好使用原始查询.

The eloquent version of this is a little bit tricky. It will be better to use a raw query in this case.

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