sql查询带有特定标签的帖子

Question

I have posts, tags, post_tags tables.

Now I want to get all posts that have (a, b, d) tags,

how do I achive that and not only getting all posts with one/two/all tags?

tables:

posts:
id, title

tags:
id, title

post_tags:
post_id, tag_id

my simple idea was this query:

select *
from post_tags pt
join posts ps on pt.post_id=ps.id
where pt.tag_id in (MY_DESIRED_TAGS)

this surely doesn't work as i get posts with only one of the tags and not all of them.

Answer 1

You can use group by and having :

select p.*
from posts p join
     post_tags pt
     on pt.post_id = p.id
where pt.tag_id in (MY_DESIRED_TAGS)
group by p.post_id
having count(*) = <# desired tags>;

The key is counting the number of tags that match. If you have three tags, then the having clause would be:

having count(*) = 3

Answer 2

SELECT *
FROM posts ps 
WHERE ps.id IN (
   SELECT pt.post_id 
   FROM post_tags pt
   WHERE pt.tag_id IN ([MY_DESIRED_TAGS])
   GROUP BY pt.post_id
   HAVING COUNT(1) = [THE_NUMBER_OF_DESIRED_TAGS]
)
;

The subquery finds the number of tags (out of the ones supplied) for every post and gives the post id values as a result set. The outer query then finds the posts with those id values. It can be done as a JOIN as other answers have indicated, but the way I have suggested might be faster since the server will not have to handle data irrelevant to the actual group operation.