Members - Average , Median Age

select
m.id as memberid,
dl.birthday as birthday,
TIMESTAMPDIFF(YEAR, str_to_date(m.birthday, '%Y%m%d'), r.start_at) AS age,
count(distinct r.id) as bookings
from members m

join driver_licenses dl
on m.id = dl.member_id

left outer join reservations r
on r.member_id = m.id


where r.state = 'completed'
and CONVERT_TZ(m.created_at, '+00:00', '+8:00') >= '2018-1-01 00:00'
and CONVERT_TZ(m.created_at, '+00:00', '+8:00') < '2019-09-01 00:00'
and m.imaginary = 'normal'
and m.state = 'normal'
and dl.birthday != 0

group by m.id

order by bookings desc