What is IS NULL doing in replacing NOT EXISTS operator with an outer join?

Question

DB version:10gR2
An example from searchoracle.target.com
The query which contains a NOT EXISTS operator
SELECT M.ModulId, M.Modul FROM MODULE M
WHERE NOT EXISTS
( SELECT PROJEKTMODUL.IdModul
  FROM PROJEKTMODUL
  WHERE M.ModulId = PROJEKTMODUL.IdModul
  AND PROJEKTMODUL.IdProjekt = 23 )is replaced by an equivalent outer join query
select distinct
       M.ModulId
     , M.Modul
  from MODULE M
left outer
  join PROJEKTMODUL
    on M.ModulId = PROJEKTMODUL.IdModul
   and PROJEKTMODUL.IdProjekt = 23
where PROJEKTMODUL.IdProjekt is nullI don't understand what
PROJEKTMODUL.IdProjekt is nullis doing in the above rewritten query.

Answer 1

It is to perform an Anti-Join. As far as I know Oracle (starting with release 10g - I thought Release 1) rewrites your NOT EXISTS and NOT IN query to a Anti Join which looks like your second query.
Look at the results of the query without the IS NULL predicate and see which records (from that set) should be selected to show the same results as the first query. The records of interest match the predicate IS NULL.