select *, (zone_requests/requests) zone_prob

from

(select rider_id, estimated_pickup_zone, count(*) zone_requests
from map600_rides_v2 where estimated_pickup_zone > 0
and estimated_dropoff_zone > 0 group by 1, 2) A

inner join

(select rider_id, count(*) requests, sum(if(status = 'COMPLETED', 1, 0)) trips
from map600_rides_v2 where estimated_pickup_zone > 0
and estimated_dropoff_zone > 0 group by 1) B
using (rider_id)

having zone_requests/requests > 0.20