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

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'