Sum of Kms /Hour

select
sum(A.mileage) as numberofkms,
A.startinghour
from
(select
ra.mileage as mileage,
CONVERT_TZ(r.start_at, '+00:00', '+8:00') as startingdate,
hour(CONVERT_TZ(r.start_at, '+00:00', '+8:00')) as startinghour,
r.id as rid

from reservations r

left outer join reservation_appendixes ra
on r.id = ra.reservation_id

where CONVERT_TZ(ra.created_at, '+00:00', '+8:00') >= '2019-3-1 00:00'
and CONVERT_TZ(ra.created_at, '+00:00', '+8:00') < '2019-4-1'
and r.state in ('completed')
group by r.id) A
group by A.startinghour