union

[slimdave]

Just to show another way of doing it ...

Select
   id,
   parent_id,
   active,
   source
From
   (
   Select
      source,
      id,
      parent_id,
      active,
      Min(source) Over
         (Partition By id) min_id_source
   From
      (
      Select
         'A' source,
         id,
         parent_id,
         active
      From
         a
      Union All
      Select
         'B',
         id,
         parent_id,
         active
      From
         b
      )
   )
Where
   source = min_id_source
/

Not suggesting that it's any better mind (I doubt that it would be), but it would be interesting to see a performance comparison of the two methods. * hint hint *