SELECT au.username, DATE(qab.created), count(*)
FROM
(
  SELECT qa.id as id, qa.question_id as qid from questions_answer qa
  GROUP BY qa.question_id
) as tablea JOIN questions_answer qab ON qab.id = tablea.id
JOIN auth_user au ON qab.creator_id = au.id
WHERE qab.created > '2012-01-01'
GROUP BY au.id, DATE(qab.created)