mysql - 在mysql中找到相关主题？

Question

我的数据库如下:

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:

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}

根据以上数据，如果输入的是数据库，那么相关主题应该按顺序显示:

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

如何编写 sql 来实现这一点？

Answer 1

您可能会找到更好的方法(条件出现两次不太好)，但是，通过连接和存在子句，您将得到您想要的。

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;

参见Sqlfiddle