Filtering from join-table

Question

I\'m having some trouble with a tricky SQL-query.

In my MySQL database there is the tables topics, tags and tags_topics to join them. I want to fetch topics that share

Answer 1

SELECT 
    topic_id
FROM
    tags_topics
WHERE
    tag_id IN (1,2,3)
GROUP BY
    topic_id
HAVING
    COUNT(*) > 2  /* or use COUNT(*) = 3 if you know that there cannot be duplicates in the junction table */