Custom badge SQL query based on how many solved topics in a certain category

Continuing the discussion from What cool badge queries have you come up with?:

I’m trying to figure out the following. I want the SQL query below to work in a single category ID:

SELECT id user_id, current_timestamp granted_at
FROM users
WHERE id  IN (
       SELECT p1.user_id
       FROM post_custom_fields pc
       JOIN badge_posts p1 ON = pc.post_id
       JOIN topics t1 ON p1.topic_id =
       WHERE p1.user_id <> t1.user_id AND
                    name = 'is_accepted_answer' AND
            p1.user_id IN (
                   SELECT user_id
                   FROM posts
                   WHERE :backfill OR IN (:post_ids)
        GROUP BY p1.user_id
        HAVING COUNT(*) > 9

Now it checks the global forum, but I want to merge it with a query like this:

SELECT DISTINCT ON (t.user_id) t.user_id, t.created_at granted_at
FROM topics t
WHERE t.category_id = 28

Any suggestions? Sorry, SQL has been a while for me.