problema count tablas diferentes misma query

[quimfv]

Código MySQL:

SELECT c.iataCompanyCode as company, 
          Sum(numArrivals) as Arrivals, 
          Sum(numDep) as Departures
FROM
(select distinct 
          a.iataCompanyCode as company, 
          count(a.idArrivalFlight) as numArrivals, 
          0  as numDep 
from `aodb`.`arrivalFlight` a 
where a.iataCompanyCode in ('SPP','IB') 
and a.scheduledTime between '2011-01-01 00:00:00' and '2011-09-01 00:00:00' 
group by a.iataCompanyCode,numDep 
union all 
select distinct b.iataCompanyCode as company, 
          0 as numArrivals, 
          count(b.idDepartureFlight) as numDep 
          from `aodb`.`departureflight` b 
     where b.iataCompanyCode in ('SPP','IB') 
     and b.scheduledTime between '2011-01-01 00:00:00' and '2011-09-01 00:00:00' 
group by b.iataCompanyCode,numArrivals )
) as c
group by c.iataCompanyCode;
 

Preguntate si esos DISTINCT son necesarios... puesto que tienen su coste...