用substring截取时间或者between

select a.course_id,
       b.course_name, 
       count(course_name) online_num
from attend_tb a
left join course_tb b
on a.course_id = b.course_id
where '19:00' between DATE_FORMAT(in_datetime,'%H:%i') and DATE_FORMAT(out_datetime,'%H:%i')
#where substring(course_datetime,10,6)>=substring(in_datetime,11,6) and substring(course_datetime,10,6)<=substring(out_datetime,11,6)
group by a.course_id,b.course_name
order by a.course_id
#Mysql日期时间格式化 %Y-%m-%d %H:%i:%S