AND field NOT IN(NULL) returns an empty set [duplicate]

Answer 1

Comparing with null is always a bad idea. Even NULL = NULL or NULL <> NULL will return NULL.

Use IS NOT NULL insetad

In your case the query is identical to:

select p.iuser_id,p.produit_id from portefeuille p
WHERE produit_id=48
AND p.iuser_id <> NULL;

and the value of p.iuser_id <> NULL will be NULL regardless of the value of p.isuer_id and this explains why it returns an empty set!