三个步骤：
①准备aa表：把试卷表和问题表union，通过uid来分组，后面用于计算活跃的月份和天数；
②准备bb表：user_info表选出6/7级的uid，后面用来与aa表连接，为的是把从未活跃过的uid也放到总表里；
③筛选计算：试卷组别（900开头）、问题组别（800开头）、年份2021 可作为count内部if筛选条件。
select 
uid, 
count(distinct date_format(b,'%Y%m')) act_month_total, 
count(distinct if(year(b)=2021,date_format(b,'%Y%m%d'),null)) act_days_2021,
count(distinct if(a>9000 and year(b)=2021,date_format(b,'%Y%m%d'),null)) act_days_2021_exam,
count(distinct if(a<9000 and year(b)=2021,date_format(b,'%Y%m%d'),null)) act_days_2021_question
from 
(
select uid, exam_id a, start_time b
from exam_record
union all
select uid, question_id a, submit_time b
from practice_record) as aa
right join
(select uid from user_info where level in (6,7)) as bb using(uid)
group by uid
order by act_month_total desc, act_days_2021 desc