SELECT
  rank_id,
  rank_title,
  IFNULL(rank_owners, "-") AS rank_owners
FROM
  phpbb3_ranks
LEFT JOIN
  (SELECT
     GROUP_CONCAT(username ORDER BY username DESC SEPARATOR ', ') AS rank_owners,
     user_rank
   FROM phpbb3_users
   GROUP BY user_rank) ro
ON user_rank=rank_id
WHERE rank_special=1;