题解 | #牛客直播各科目平均观看时长#

select 
    course_name,
    round(avg(times),2)  as avg_len
from (select 
    c.course_name,
    TIMESTAMPDIFF(MINUTE, a.in_datetime, a.out_datetime) as times
from course_tb as c join attend_tb as a on
c.course_id = a.course_id) t group by course_name order by avg_len desc;

重点在于求两个时间的差 timestampdiff函数