SELECT
  submit_month,
  COUNT(question_id) month_q_cnt,
  ROUND(COUNT(question_id) / day, 3) avg_day_q_cnt
FROM
  (
    SELECT
      DATE_FORMAT(submit_time, '%Y%m') submit_month,
      question_id,
      DAY(LAST_DAY(submit_time)) day
    FROM
      practice_record
    WHERE
      DATE_FORMAT(submit_time, '%Y') = 2021
  ) tb
GROUP BY
  submit_month,
  day
UNION
SELECT
  '2021汇总' submit_month,
  COUNT(question_id) month_q_cnt,
  ROUND(COUNT(question_id) / 31, 3) avg_day_q_cnt
FROM
  practice_record
WHERE
  DATE_FORMAT(submit_time, '%Y') = 2021
ORDER BY
  submit_month;