用多个联接计数

Question

Assuming you have 3 tables: posts , tags , post_tag . There's an m:n relationship between the posts and tags , post_tag is used to connect them.

How would you count the number of posts that have a particular pair of tags? For instance, counting the number of posts that are tagged with both "news" and "featured"?

Answer 1

Simply use EXISTS for it

select count(*)
from posts
where exists(select 1 from post_tag 
             join tag on post_tag.tid = tag.tid
             where post_tag.pid = posts.pid and tag.name = 'news') and
      exists(select 1 from post_tag 
             join tag on post_tag.tid = tag.tid
             where post_tag.pid = posts.pid and tag.name = 'featured')

Answer 2

This is a case of relational division . There are many ways to solve it.
Assuming a standard many-to-many implementation , this should be among the fastest and simplest:

SELECT count(*)
FROM   post_tag pt1
JOIN   post_tag pt2 USING (post_id)
WHERE  pt1.tag_id = (SELECT tag_id FROM tags WHERE tag = 'news')
AND    pt2.tag_id = (SELECT tag_id FROM tags WHERE tag = 'featured');

That's counting posts with at least the two given tags. There can be more tags for the same post.

We have assembled an arsenal of techniques under this related question:

• How to filter SQL results in a has-many-through relation