select, where and null-values (or: select null<>'1' is fuzzy)

I've a problem when selecting values out a table.

manana=# select * from test;
l1 | l2 | l3
----+----+----
1 | 2 | 3
2 | 3 | 4
3 | 4 | 5
4 | 5 | 6
| 5 | 6
(5 rows)

where l1 is NULL in the last line.
now I do

manana=# select * from test where l1<>1;
l1 | l2 | l3
----+----+----
2 | 3 | 4
3 | 4 | 5
4 | 5 | 6
(3 rows)
and dont get the line with the null-entry !
In fact the problem is also describeable like this:

manana=# select NULL<>'1';
?column?
----------

(1 row)

This gives neither true nor false.

Why is this like this and how can I work around ?

thnx,
peter

--
mag. peter pilsl

phone: +43 676 3574035
fax : +43 676 3546512
email: pilsl(at)goldfisch(dot)at
sms : pilsl(at)max(dot)mail(dot)at

pgp-key available