gistfile1.sql

SELECT AVG(i.total), MAX(i.total)
FROM(
  SELECT COALESCE(SUM(a.count) + SUM(b.count), 0) as total
  FROM users
  LEFT JOIN (
    SELECT 
      COUNT(*) as count,
      user_id
    FROM "quick_answers" 
    GROUP BY user_id
  ) a ON a.user_id = users.id
  LEFT JOIN (
    SELECT 
      COUNT(*) as count,
      taken_exams.user_id
    FROM "answers" 
    INNER JOIN taken_exams ON taken_exams.id = answers.taken_exam_id
    GROUP BY taken_exams.user_id
  ) b ON b.user_id = users.id
  GROUP BY users.id
) i
WHERE i.total != 0;