题解 | #牛客直播各科目出勤率#

这题的难点在于怎么链接，要表现出所有用户，因此behavior表是base表，再左连接，才不会丢失用户信息

select be.course_id,course_name,
       round(100*sum(if(sum_time>=10,1,0))
       /sum(if(if_sign=1,1,0)),2) as attend_rate
from   behavior_tb be left join 
(select user_id,course_id,
      sum(timestampdiff(minute,in_datetime,out_datetime)) as sum_time
      from attend_tb
      group by user_id,course_id
) att     using(user_id,course_id)
         left join course_tb cou on be.course_id=cou.course_id
group  by be.course_id,course_name