Median Function - Shiva

set @startDate = "2020-02-20 00:00:00";
set @monthInterval = 2;
select
    round(avg(dur),2) as median_val
from (
select
    dur,
    @rownum:=@rownum+1 as `row_number`,
    @total_rows:=@rownum
from (
select
    r.id as rid,
    c.id as cid, c.car_class_id as ccid,
    r.start_zone_id as zid, z.region as region, z.city as team,
    convert_tz(r.return_at, '+00:00', '+8:00') as return_at,
    timestampdiff(minute, r.start_at, r.end_at)/60 as dur,
    cc.car_name as model,
    m.id as mid
from cars c, reservations r, members m, zones z, car_classes cc
where c.id = r.car_id
    and m.id = r.member_id
    and z.id = r.end_zone_id
    and c.car_class_id = cc.id
    and z.id not in (2,3,101)
    and z.city = 'PG'
    and m.imaginary in ('normal', 'sofam')
    and r.state in ('completed')
    and convert_tz(r.return_at, '+00:00', '+8:00') >= @startDate
    and convert_tz(r.return_at, '+00:00', '+8:00') < date_add(@startDate, interval @monthInterval month)
) as temp, (select @rownum:=0) r
order by dur
) as temp2
where temp2.row_number IN ( floor((@total_rows+1)/2), floor((@total_rows+2)/2) );