Mysql GROUP_CONCAT of first n rows || Selecting first and last values in a group || Select first and last row from group_concat when grouping sortable days

select s.id,count(e.id) as total,
substring_index(group_concat(e.id order by e.id asc),',',5) as group_value
from students s left join exams e on s.id=e.student_id
group by s.id
order by count(e.id) desc