如何只从第二个表中查找最后一行，以及它的第一个表中的行 [英] How to find only last row from second table, along with its first table row

问题描述

我有两个桌子. 期刊和卷. 日记表具有唯一的行，卷表具有基于日记表ID的行，名称为journal_id(可以为多个).

I have two tables. journals and volumes. Journal Table has unique rows, and volumes table have rows based on journal table id, name is journal_id(may be multiple).

期刊表为:

id | journal_name

1  | journal1
2  | journal2

体积表为:

id | journal_id | volume_name

1  |    1       |  volume1
2  |    1       |  volume2
3  |    1       |  volume3
4  |    2       |  volume4
5  |    2       |  volume5

现在，我需要与日志表中的行连接，并且仅基于journal_id的卷的最后一行.

Now I need join with row from journal table and only last rows of volumes based on journal_id.

结果应为:

id | journal_name | journal_id | volume_name

1  | journal1     |   1        |   volume3
2  | journal2     |   2        |   volume5

不是卷表中的所有行. (只需要每个journal_id组的最后一行).

从mysql查询获得的结果是:

SELECT J.journal_name,V.id,V.journal_id FROM journals AS J 
INNER JOIN (SELECT *
FROM volumes
WHERE id IN (
SELECT MAX(id)
FROM volumes
GROUP BY journal_id
)) AS V ON J.id = V.journal_id

现在我在laravel中的尝试是:

控制器为:

public function index()
{
$volumes = volume::with('volumes')->orderBy('id','desc')->limit(1)->get();
    return view('welcome',compact('volumes'));
}

体积模型为:

function volumes()
 {
      return $this->belongsTo(journal::class, 'journal_id');
 }

但是从整个卷表中只给出一行.我需要卷表中每一组journal_id的最后一行.

But it is giving only one row from entire volume table. I need last one row from each group of journal_id in volume table.

推荐答案

您可以使用HasOne关系:

class Journal extends Model
{
    public function latestVolume()
    {
        return $this->hasOne(Volume::class)->orderByDesc('id');
    }
}

$journals = Journal::with('latestVolume')->get();

请注意，这仍然会从数据库中获取所有卷.然后，它将丢弃旧"的那些.

Be aware that this will still fetch all volumes from the database. It then discards the "old" ones.

如果仅通过获取最新卷来提高性能，可以使用我创建的以下程序包:

If you want to improve the performance by really only fetching the latest volume, you can use this package I've created: https://github.com/staudenmeir/eloquent-eager-limit

该程序包允许您将limit()应用于该关系:

The package allows you to apply limit() to the relationship:

class Journal extends Model
{
    use \Staudenmeir\EloquentEagerLimit\HasEagerLimit;

    public function latestVolume()
    {
        return $this->hasOne(Volume::class)->orderByDesc('id')->limit(1);
    }
}

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