Laravel 按类别 ID 列出的最新新闻答案

【问题标题】：Laravel last news item by category idLaravel 按类别 ID 列出的最新新闻
【发布时间】：2020-12-02 03:48:21
【问题描述】：

我正在尝试了解按类别 ID 分组的最新新闻项目的概览。

示例数据：

+----+--------+-------------+-------------------------+-------------------------+
| id | title  | category_id |       created_at        |       updated_at        |
+----+--------+-------------+-------------------------+-------------------------+
|  1 | Item 1 |           1 | 2020-08-01 12:00:00.000 | 2020-08-01 12:00:00.000 |
|  2 | Item 2 |           2 | 2020-08-02 12:00:00.000 | 2020-08-02 12:00:00.000 |
|  3 | Item 3 |           4 | 2020-08-02 12:00:00.000 | 2020-08-02 12:00:00.000 |
|  4 | Item 4 |           1 | 2020-08-04 12:00:00.000 | 2020-08-04 12:00:00.000 |
|  5 | Item 5 |           2 | 2020-08-11 12:00:00.000 | 2020-08-11 12:00:00.000 |
+----+--------+-------------+-------------------------+-------------------------+

我想要输出：

+----+--------+-------------+-------------------------+-------------------------+
| id | title  | category_id |       created_at        |       updated_at        |
+----+--------+-------------+-------------------------+-------------------------+
|  3 | Item 3 |           4 | 2020-08-02 12:00:00.000 | 2020-08-02 12:00:00.000 |
|  4 | Item 4 |           1 | 2020-08-04 12:00:00.000 | 2020-08-04 12:00:00.000 |
|  5 | Item 5 |           2 | 2020-08-11 12:00:00.000 | 2020-08-11 12:00:00.000 |
+----+--------+-------------+-------------------------+-------------------------+

Laravel Eloquent 有没有简单的方法来做到这一点？

【问题讨论】：

这能回答你的问题吗？ Laravel: How to get last N entries from DB

标签： laravel eloquent greatest-n-per-group

【解决方案1】：

您将需要一个自加入新闻表来选择每个类别的最新新闻，例如

select n.*
from news n
left join news n1 on n.category_id = n1.category_id
and n.created_at < n1.created_at
where n1.category_id is null

DEMO

使用查询生成器，您可以将其重写为

DB::table('news as n')
  ->select('n.*')
  ->leftJoin('news as n1', function ($join) {
        $join->on('n.category_id', '=', 'n1.category_id')
             ->whereRaw(DB::raw('n.created_at < n1.created_at'));
   })
  ->whereNull('n1.category_id')
  ->get();

【讨论】：