具有两个 INNER JOIN 的 MySQL 查询在结果中返回重复条目

Question

我有以下数据结构：文章有 m:n 个用户

共有三个表：articles、users 和 articles_users（链接表）

现在我需要这个查询：给我所有最近写过文章的用户。

不幸的是，这个查询返回了重复的结果：

SELECT  DISTINCT users.id, 
        users.username, 
        articles.published, 
        articles_users.user_id
FROM    users 
        INNER JOIN articles_users 
            ON users.id = articles_users.user_id
        INNER JOIN articles     
            ON articles.id = articles_users.article_id
ORDER BY articles.published DESC
LIMIT 25;

结果：

id      username        published   user_id
113     silva_mihat     2012-10-30  112
228     paula_tille     2012-10-27  258
228     paula_tille     2012-10-26  258
631     andrea_gurkow   2012-10-24  631
275     hubert_mayer    2012-10-24  275
198     annette_mulger  2012-10-22  198
255     uta_zuffter     2012-10-22  235
and so on ...

有人知道为什么 DISTINCT 在这里不起作用吗？

Answer 1

因为DISTINCT 适用于整行（不仅仅是users.id 本身）。如您所见，所有返回的行都不是唯一。尝试这样的事情，子查询背后的想法是它为每个article_id获取最近的published日期

SELECT  users.id, 
        users.username, 
        articles.published
FROM    users 
        INNER JOIN articles_users 
            ON users.id = articles_users.user_id
        INNER JOIN articles 
            ON articles.id = articles_users.article_id
        INNER JOIN 
        (
            SELECT id, MAX(published) maxDate
            FROM articles
            GROUP BY id
        ) c ON  articles.id = c.ID AND 
                articles.published = c.maxDATE
-- ORDER BY articles.published DESC
-- LIMIT 25

Answer 2

这应该按作者而不是按文章分组。

select
  users.id,
  users.username,
  maxPublished
from users
inner join (
  select
    max(articles.published) as maxPublished,
    articles_users.user_id as userID
  from articles
  join articles_users on articles_users.article_id = articles.id
  group by articles_users.user_id
) as p on users.id = userID
order by maxPublished desc
limit 25
;