SQL 获取具有与特定值匹配的最新关联的记录

Question

上下文

使用 PostgreSQL，我正在存储 articles 及其关联的 events。为了示例，这里是 2 table 的简单结构：

文章table

+----+-----------+-----------------------+
| ID |   title   |      description      |
+----+-----------+-----------------------+
|  1 | article 1 | article 1 description |
|  2 | article 2 | article 2 description |
|  3 | article 3 | article 3 description |
|  4 | article 4 | article 4 description |
|  5 | article 5 | article 5 description |
+----+-----------+-----------------------+

事件table

+-----+-----------+--------------+----------------+--------------+
| ID  |   name    | eventable_id | eventable_type | created_at   |
+-----+-----------+--------------+----------------+--------------+
|  1  | validated |            1 | Article        | 2020-05-10   |
|  2  | reported  |            1 | Article        | 2020-05-11   |
|  3  | reported  |            2 | Article        | 2020-05-10   |
|  4  | reported  |            2 | Article        | 2020-05-11   |
|  5  | reported  |            2 | Article        | 2020-05-12   |
|  6  | reported  |            3 | Article        | 2020-05-20   |
|  7  | validated |            3 | Article        | 2020-05-21   |
|  8  | reported  |            4 | Article        | 2020-05-12   |
|  9  | moved     |            4 | Article        | 2020-05-13   |
|  10 | reported  |            4 | Article        | 2020-05-14   |
|  11 | moved     |            5 | Article        | 2020-05-13   |
|  12 | moved     |            5 | Article        | 2020-05-14   |
+-----+-----------+--------------+----------------+--------------+

问题

在这里，我需要能够获得所有 articles，它们的最新 events 是 reported。

所以比如按照上面的数据，我们只能得到：

• article 1:因为已经validated然后reported
• article 2: 因为只reported
• article 4：因为已经moved然后reported（同第1条）

如您所见：

• article 3: 不应该是 return 因为它的最新事件是 validated.
• article 5: 不应该是 return 因为它的最新事件是 moved.

我可以轻松找到所有 articles 都有 reported 事件。但是如何让最近一次活动的人成为 reported 呢？

以下是我到目前为止所做的尝试，但没有成功：

SELECT *
FROM articles a
INNER JOIN (
    SELECT *
    FROM events
    WHERE name = 'reported'
    ORDER BY created_at
    LIMIT 1
) AS e ON e.moderable_id = a.id AND e.moderable_type = 'Article'

我们目前有：

• 459 892 articles
• 62074events

Answer 1

您可以使用相关子查询进行过滤：

select a.*
from articles a
where 
    (
        select e.name
        from events e
        where e.eventable_id = a.id and e.eventable_type = 'Article'
        order by created_at desc 
        limit 1
    ) = 'reported'

我们也可以用横向连接来表达：

select a.*
from articles a
inner join lateral (
    select e.name 
    from events 
    where e.eventable_id = a.id and e.eventable_type = 'Article'
    order by created_at desc 
    limit 1
)  x on x.status = 'reported'