Null in the where-clause

Hello everybody,

I've got a problem with the "null"-statement within the where-clause.
I installed the postgres-server 7.2.1 on a Suse Linux machine.
Then I imported an existing databasedump.
But when I make the following query:
select * from tbl_categories where categoryid=null;
I get 0 rows as result, but there are several records, which
corrsponds to my query.
What could be the problem? Do I have to reinstall the server
with a special parameter?

Thanx in advance
Michael Walzl