MySQL 排序的分组依据 table

Question

我有两个具有这些结构的表：

Table 1（评论）

+----+---------+------------+--------+---------+
| id | post_id | created_at | author | content |
+----+---------+------------+--------+---------+
|  1 |       3 | 03-01-2020 | Jack   | XXX     |
|  2 |       1 | 02-29-2020 | Joe    | XXX     |
|  3 |       3 | 02-24-2020 | Anna   | XXX     |
|  4 |       2 | 03-01-2020 | Mia    | XXX     |
+----+---------+------------+--------+---------+

Table 2 (posts)

+-----+-------------+---------------------+---------+
| id  | category_id |        name         | content |
+-----+-------------+---------------------+---------+
|   1 |           1 | some random name    | xxxxxxx |
|   2 |           2 | another random name | xxxxxxx |
|   3 |           1 | third random name   | xxxxxxx |
+-----+-------------+---------------------+---------+

我的目标是获得一个列表，其中包含来自特定 category_id 的每个 post 的最新评论。

因此，例如，如果我将类别 1 作为输入，我需要 ID 为 2 和 1 的评论，因为这些始终是给定类别的每个 post 的最后评论。

到目前为止，我得到了这个查询，其中 returns 所有条目都按创建日期排序。

select *
from comments c
join posts p on c.post_id = p.id
where p.category_id = 1
order by c.created_at desc;

我已经阅读了有关 group by 属性的信息，但这并不能保证我能获得最后的条目。我该如何解决这个问题？

Answer 1

假设 Table2 是类别 table，如果您只想在结果中包含 Table1 的列，那么您不需要不需要加入。
使用 NOT EXISTS:

select t1.* 
from table_1 t1
where not exists (
  select 1 from table_1
  where table2_id = t1.table2_id and created_at > t1.created_at
)

或相关子查询：

select t1.* 
from table_1 t1
where t1.created_at = (select max(created_at) from table_1 where table2_id = t1.table2_id)

编辑：

select c.*
from posts p inner join (
  select c.* from comments c
  where not exists (
    select 1 from comments
    where post_id = c.post_id and created_at > c.created_at
  )
) c on c.post_id = p.id
where p.category_id = ?

参见demo。
或者：

select c.*
from posts p inner join (
  select c.* from comments c
  where c.post_id in (select id from posts where category_id = ?)
  and not exists (
    select 1 from comments
    where post_id = c.post_id and created_at > c.created_at
  )
) c on c.post_id = p.id

参见demo。
将 ? 替换为您要搜索的 category_id。
结果：

| id  | post_id | created_at          | author | content |
| --- | ------- | ------------------- | ------ | ------- |
| 1   | 3       | 2020-03-01 00:00:00 | Jack   | XXX     |
| 2   | 1       | 2020-02-29 00:00:00 | Joe    | XXX     |

Answer 2

将 GROUP BY 与聚合函数 MAX 结合使用应该可以解决您的问题：

SELECT 
  b.id AS category_id, 
  MAX(a.created_at) AS last_created
FROM table_1 a
JOIN table_2 b on a.table2_id = b.id
GROUP BY b.id
ORDER BY last_created DESC;

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