difference between EXCEPT and NOT IN?

Hi,

The 2 following statements don't give the same result. I expected the
second ti give the exact same result as the first one.
What am I missing?

development=> SELECT id FROM entrees  except  select entree_id from
postes ORDER BY id desc; id
------3651
(1 row)
development=> SELECT id FROM entrees WHERE  id not in (select
entree_id from postes) ORDER BY id desc;id
----
(0 rows)


thanks in advance for the help.

Raph
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