Oracle Nested Query with Group By and Having Clauses
If I understand correctly:

select *
  from (SELECT D.DID, AVG(F.DIST) as AvgDist 
        from Destinations D 
        join Flights F ON F.DID = D.DID 
        GROUP BY D.DID) avgdist
  join
       (SELECT F.AID,F.DID, SUM(F.DIST) as fDist 
        from Flights F 
        join Destinations D ON F.DID = D.DID 
        GROUP BY F.AID,F.DID) sumdist
        on avgdist.did = sumdist.did
  where avgdist.avgdist &lt; sumdist.fDist;