with t1 as( select course_id,course_name, in_datetime as mytime, 1 as uv from attend_tb join course_tb using(course_id) union all #不能省略all select course_id,course_name, out_datetime as mytime, -1 as uv from attend_tb join course_tb using(course_id) ), t2 as( select course_id,course_name,mytime, sum(...