Oracle:

SELECT h.hacker_id, h.NAME FROM submissions s JOIN challenges c ON s.challenge_id = c.challenge_id JOIN hackers h ON s.hacker_id = h.hacker_id JOIN difficulty d ON c.difficulty_level = d.difficulty_level WHERE s.score = d.score GROUP BY h.hacker_id, h.NAME HAVING Count(s.challenge_id) > 1 ORDER BY count(s.challenge_id) DESC, h.hacker_id ;