子查询找出每门课程总报名人数简单易懂

子查询计算每门课程的总人数，(select count(distinct user_id) from behavior_tb as t2 where t2.course_id=t3.course_id and if_sign=1 group by t2.course_id )

出勤人数：count(distinct if(timestampdiff(minute,in_datetime,out_datetime)>=10,t3.user_id,null))

然后只用把表1和表3join就可以了：course_tb as t1 join attend_tb as t3 using(course_id)

select
    course_id,
    course_name,
    round(100*(count(distinct if(timestampdiff(minute,in_datetime,out_datetime)>=10,t3.user_id,null))/(select count(distinct user_id) from behavior_tb as t2 where t2.course_id=t3.course_id and if_sign=1 group by t2.course_id )),2) as 'attend_rate(%)'
from course_tb as t1 
    join attend_tb as t3 using(course_id)
group by course_id,course_name
order by course_id