Strange result with select/not null/OR

Hi all!

I get a strange result with this query:

SELECT count(*)
FROM CONT_CAT
WHERE id_instal = 2
and id_cat = 2
and (sp_link IS NOT NULL OR sp_link <> '')

I have one row with an empty value ('') on sp_link, so, when I execute
the query expect 0 on the result.
I don't know why the result is 1.
If I only put the last part ("sp_link <> ''") everithing goes ok.
But I need to check both conditions, IF is null and IF is empty. And I
can't understand why doesn't work.

Any ideas?

Thanks in advance!

--
Fernando O. Papa
DBA