给大家乐一乐

with a1 as(select
    a.pmonth,
    sum(a.amount) sum1
from
    (
        select
            month (submit_time) pmonth,
            uid,
            count(distinct (day (submit_time))) amount
        from
            exam_record
        where
            year (submit_time) = 2021
        group by
            pmonth,
            uid
    ) a
group by
    a.pmonth
)

select month,round(a1.sum1 / mau, 2) avg_active_days,mau

from 
(select concat(
           year (e1.submit_time), if(
            month (e1.submit_time) < 10, concat(0, month (e1.submit_time)),
            month (e1.submit_time)
        )
           ) month,
           month(e1.submit_time) month1,
       count(distinct e1.uid)                    mau
       
from exam_record e1
where
    year (e1.submit_time) = 2021
group by    month,month1
) b

    inner join a1 on b.month1=a1.pmonth