在mysql中查找相关主题? [英] Find related topics in mysql?

问题描述

我有如下数据库:

Post {id, title}
Topic {id, name}
Post_Topic {PostId, TopicId}

由于每个帖子可能有很多主题，所以我想根据它们一起出现在帖子中的次数来获取相关主题.例如:

Since each post may have many topics, so I want to get related topics based on the number of times they appears in posts together. For example:

post 1 has topics {database, mysql, mobile}
post 2 has topics {database, mysql, android}
post 3 has topics {database, mysql, algorithm}
post 4 has topics {database, algorithm, web programming}

根据上面的数据，如果输入的是database，那么相关主题应该是按顺序显示的:

Based on the above data, if the input is database, then related topics should be displayed in order:

mysql (appears 3 times with database)
algorithm (appears 2 times with database)
android
mobile

我该如何编写 sql 来实现这一点?

How can I write the sql to achieve that?

推荐答案

您可能会找到更好的方法(有两次条件不是很好)，但是，使用 join 和 exists 子句，您将得到您想要的想要.

You may find a better way (not great to have a condition two times), but, with a join and an exists clause, you'll get what you want.

select t_id, t.title, count(*) as cnt
from post_topic pt
join topic t on t.id = pt.t_id

where exists (select null
              from post_topic pt1
              join topic t1 on pt1.t_id = t1.id
              where t1.title = 'database'  and p_id = pt.p_id)
and t.title <> 'database'
group by  t_id, t.title
order by cnt desc;

见Sqlfiddle

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