i think u miss understand about table relation..
users : scores = 1 : *
just join
is not a solution.
is this your intention?
SELECT users.username, avg(scores.point), avg(scores.avg_time)
FROM scores, users
WHERE scores.user_id = users.id
GROUP BY users.username
ORDER BY avg(scores.point) DESC, avg(scores.avg_time)
LIMIT 0, 20
(this query to get each users average point and average avg_time by desc point, asc )avg_time
if you want to get each scores ranking? use left outer join
SELECT users.username, scores.point, scores.avg_time
FROM scores left outer join users on scores.user_id = users.id
ORDER BY scores.point DESC, scores.avg_time
LIMIT 0, 20