select date_format(submit_time, '%Y%m') as month,
 round(
 count(distinct uid, date_format(submit_time, '%Y%m%d')) / count(distinct uid), 2) as avg_active_days,
 count(distinct uid) as mau
from exam_record
where submit_time is not null and year(submit_time) = 2021
group by month

很怪，为什么第三行还要用一次date_format函数？？