Right, my bad. Upper parent was about using IN and having return everything "but" 5 and 6. So here the select that will do that:
select mumu from kaka where mumu not in (5, 6) or mumu is null.
That will return everything, null included, except for 5 and 6.
But wait, there is more. If, for example, performance is the main issue here above query is quite slow. Even on an indexed table on column mumu, it will still do a full scan of the table before returning. How to improve performance in this case? Well, you use LEFT join on itself. Implementation is left as exercise for reader :D.
