我怎样才能 select 每个主题的最高 post 并在给定此模式设计的情况下将它们关联起来？

Question

我正在为一个网站开发一个小型论坛组件，我正在创建一个页面，我想在其中显示每个主题及其评分最高的答案。表格如下所示：

POST       USER      TOPIC
id         id        id 
date       name      title   
text       bio       date
views                
likes
topic_id
author_id

我的查询是这样的：

select 
      u.id, u.name, u.bio, 
      p.id, p.date, p.text, p.views, p.likes, 
      t.id, t.title, t.date
 from
      ( select p.id, max(p.likes) as likes, p.topic_id 
        from post as p group by p.topic_id ) as q 

      inner join post as p on q.id = p.id
      inner join topic as t on t.id = q.topic_id 
      inner join user as u on u.id = p.author_id

 order by date desc;

我运行遇到的问题之一是"q"。 Postgresql 不会让我运行 "q" 查询，因为它希望 "p.id" 在 "group by" 子句或聚合函数中。我尝试使用 "distinct on (p.id)" 但我收到相同的错误消息：p.id must appear in the GROUP BY clause or be used in an aggregate function.

没有 p.id 属性，我无法将它有意义地 link 到其他表；还有其他方法可以实现吗？

Answer 1

;WITH cte AS (
    SELECT
       u.id AS UserId
       ,u.name
       ,u.bio
       ,p.id AS PostId
       ,p.[date] AS PostDate
       ,p.text
       ,p.views
       ,p.Likes
       ,t.id AS TopidId
       ,t.title
       ,t.[date] AS TopicDate
       ,p.Likes
       ,ROW_NUMBER() OVER (PARTITION BY t.id ORDER BY p.Likes DESC, p.[date] DESC) AS RowNum
       ,DENSE_RANK() OVER (PARTITION BY t.id ORDER BY p.Likes DESC) AS RankNum
    FROM
       topic t
       INNER JOIN post p
       ON t.id = p.topic_id
       INNER JOIN [user] u
       ON p.author_id = u.id
)

SELECT *
FROM
    cte
WHERE
    RowNum = 1

;

如果您想查看最喜欢的人的关系，请将 RowNum 切换到 RankNum

Answer 2

这是一个常见的需求：分组时，显示每个组的 first/last a 按其他一些标准 b 排名。我没有它的名字，但是 this seems to be the canonical question. You can see there are a lot of choices! My favorite solution is probably a lateral join:

SELECT  u.id, u.name, u.bio,
        p.id, p.date, p.text, p.views, p.likes,
        t.id, t.title, t.date
FROM    topic t
LEFT OUTER JOIN LATERAL (
  SELECT  *
  FROM    post
  WHERE   post.topic_id = t.id
  ORDER BY post.likes DESC
  LIMIT 1
) p
ON true
LEFT OUTER JOIN "user" u
ON  p.author_id = u.id
;

Answer 3

SELECT
  u.id AS uid, u.name, u.bio 
  , p.id AS pid, p."date" AS pdate, p.text, p.views, p.likes
  , t.id AS tid, t.title, t."date" AS tdate
FROM post p  
JOIN topic t ON t.id = p.topic_id 
JOIN user u ON u.id = p.author_id
WHERE NOT EXISTS ( SELECT *
    FROM post nx
    WHERE nx.topic_id = p.topic_id
    AND nx.likes > p.likes)
ORDER BY p."date" DESC
    ;

我怎样才能 select 每个主题的最高 post 并在给定此模式设计的情况下将它们关联起来？

How can I select the highest post per topic and associate them given this schema design?

sql

postgresql

greatest-n-per-group