题解 | #月总刷题数和日均刷题数#

select
    b.submit_month,
    count(question_id) as month_q_cnt,
    round(count(question_id)/day,3) as avg_day_q_cnt
from practice_record a
left join (
    select 
    id,
    concat(substring(submit_time,1,4),substring(submit_time,6,2)) as submit_month,
    day(last_day(submit_time)) as day  
    from practice_record) b
on a.id=b.id
where substring(submit_time,1,4) ='2021'
group by b.submit_month,b.day

union all 

select 
    '2021汇总',
    count(id),
    round(count(id)/31,3) from practice_record
where substring(submit_time,1,4) ='2021'
order by submit_month