Query to find posts with an exact set of tags (many-to-many relationship)

Question

Suppose I have the following three tables expressing a relationship where posts are given tags (a many-to-many relationship):

create table posts (id integer, con         


        

Answer 1

Should be able to do this use NOT EXISTS, e.g.,

select t1.post_id
from post_tags as t1
    inner join post_tags as t2 on t2.post_id = t1.post_id
where 
    t1.tag = 'clever' 
and t2.tag = 'interesting' 
and not exists (
    select *
    from post_tags t3
    where 
        t3.tag not in ('clever', 'interesting')
    and t3.post_id = t1.post_id
)

Answer 2

This is an exact relational division problem.

In SQL Server a well performing method (assuming unique constraint on post_id,tag) is

SELECT post_id
FROM   post_tags
GROUP  BY post_id
HAVING MIN(CASE
             WHEN Keyword IN ( 'clever', 'interesting' ) THEN 1
             ELSE 0
           END) = 1
       AND SUM(CASE
                 WHEN Keyword IN ( 'clever', 'interesting' ) THEN 1
                 ELSE 0
               END) = 2  

So I wouldn't rule out the idea of using GROUP_CONCAT in the HAVING instead.

HAVING GROUP_CONCAT(DISTINCT Keyword ORDER BY Keyword) = 'clever,interesting'