SELECT
  t.hacker_id,
  t.name,
  SUM(mscore) AS total
FROM (SELECT
  h.hacker_id,
  h.name,
  s.challenge_id,
  MAX(score) AS mscore
FROM Hackers AS h
JOIN Submissions AS s
  ON h.hacker_id = s.hacker_id
GROUP BY h.hacker_id,
         h.name,
         s.challenge_id) AS t
GROUP BY t.hacker_id,
         t.name
HAVING total != 0
ORDER BY total DESC, t.hacker_id;