Customise current view

[JM]

I have a number of categories set up on my outlook.
However I don't want the appointments in 3 of these
categories to be visible at certain times. The ones I do
want visible are those with the default null value in
the categories field.

I created a filter under view, current view, customise
current view, filter, more choices tab, and
inserted 'personal' which is 1 of the
categories that I didn't want visible. I then went into
the SQL tab and changed the syntax from

("DAV:isfolder" = false AND "DAV:ishidden" = false) AND
("urn:schemas-microsoft-comfficeffice#Keywords"
= 'Personal')
to
("DAV:isfolder" = false AND "DAV:ishidden" = false) AND
("urn:schemas-microsoft-comfficeffice#Keywords"
<> 'Personal')

- this worked by displaying everything in my outlook
except for appointments with personal as the
category. I then tried to add another category I wanted
omitting using the following syntax:

("DAV:isfolder" = false AND "DAV:ishidden" = false) AND
(("urn:schemas-microsoft-comfficeffice#Keywords"
<> 'Personal' OR "urn:schemas-microsoft-
comfficeffice#Keywords" <> 'Birthdays'))

which Didn't work and everything was displayed. It was
suggested I change the OR to and AND which also didn't
work because then nothing showed in my calendar at all.

As mentioned previously, the ones I do want visible are
those with the default null value in
the categories field.

I hope someone can help?