Understand Query

Hello

  I need to understand why those queries doesn't send the same result :

-- TAB2 contains rows with ref_id_tab1 null

   select count(*)

  FROM TAB1

  where id  not in  ( select ref_id_tab1

	FROM TAB2
	where ref_id_tab1  is not null);

  COUNT(*)

----------

5

select count(*)

  FROM TAB1

  where id  not in  ( select ref_id_tab1

	FROM TAB2
	where ref_id_tab1);

 

   COUNT(*)

----------

0 Regards Djm