Viewing categories

[Jo]

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

blah blah blah = 'personal'
to
blah blah blah <> '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:

blah blah blah <> 'Personal' OR blah blah <> 'Birthdays'

which Didn't work and everything was displayed. So i
tried .....

blah blah blah <> 'Personal' AND blah blah <> 'Birthdays'

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.

Does anyone know how to set this up so it says

"show me everything with a NULL in this field"
or
"show me everything with is not = 'X' and not = 'Y' and
not = 'Z'.