题解 | distinct关键字后面加两个字段，可以去重组合

# distinct关键字后面也可以加if()函数，再结合count()使用则可以实现指定条件下的计数操作
# 注意活跃天数是各个用户的不同上线日子总数之和
select date_format(start_time, '%Y%m') month,
round(count(distinct uid, if(submit_time is not null, date_format(submit_time, '%Y%m%d'), null))/count(distinct if(submit_time is not null, uid, null)), 2) avg_active_days,
count(distinct if(submit_time is not null, uid, null)) mau
from exam_record
where date_format(start_time, '%Y') = '2021'
group by date_format(start_time, '%Y%m');