SQL - find 'missing' numbers

I am trying to find a way in SQL to get a list of all values that do not exist in a field. As in...

select "value"
where "value" not in (select "number" from file)

It's more complex than that, but if I can get the basics then I should be able to expand it.

Let's say I have a file that has a field "number". It's a 3-digit field. I know that over 800 of the 999 possible (non-zero) values are already used. I need to know which values are not used.

I've tried using CREATE SEQUENCE and it creates a Data Area (with junk in it).

I suppose I could create a file of nothing but sequential numbers from 1 to whatever, but I'm hard-headed and this really shouldn't be that hard.

Am I missing something obvious? I can probably get around this particular issue, but I'd really like to have a new technique that I can use in the future.

Thanks in advance,

Tim