The Null Value -- different results in Sql syntaxes

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I have 2 different queries (with different purposes) working perfectly but there is something I can not grasp yet about the Null value

Here is one query:

 select NCLI , LOCALITE , CAT 
 from CLIENT
 where CAT not in (’B1’,’C1’)
 or LOCALITE not in (’Lille’,’Namur’);

It shows exactly what I want Including the rows where CAT has a null value. Now this seems perfectly logical to me.

The only thing that disturbs me is that the query below only worked when I specified I did wanted the null rows in CAT to show up

 select NCLI , LOCALITE , CAT from CLIENT where CAT <> ’C1’ or CAT is null or LOCALITE = ’Namur’;

When I don't add -- or CAT is null -- the null values don't show up. Could someone tell me what I'm not grasping here yet? Thanks.

asked 16 secs ago

KastelA Kastel

3

The Null Value -- different results in Sql syntaxes