Re: cardinality of 1

Jonathan Leffler wrote:

> The 'nullary key' is a key on an empty set of columns. At most one row
> can exist ...

Thanks very much for your comment. I think I was confused about this 'nullary key' term, which I hadn't noticed outside this group, somehow associating it with ALL tables, which doesn't seem to make sense now.

> I'm not really sure what the second half of your last sentence means.
> If you're looking for a 'cardinality in any amount' where the amount is
> not (zero or) one, then the 'nullary key' is not the answer.

I guess I am merely looking for useful applications of 'implied' cardinality, if I may call it that.

> Going back to BB's comment (as quote above), I don't think that an RVA
> is needed to permit a nullary key...

It doesn't seem necessary but I wonder if it would have a use.

> Any table (including DEE and DUM)can have a nullary key.

Could this include a view of a table that doesn't?

>
> You're original source for information on empty keys (nullary keys)
> would be in Date's "Writings" series...

Found one source in '1991-1994' ('installment 19' - it is thought-provoking.) Gave '1989-1991' to a far-away friend - will see if he can send it. (Funny how I obviously didn't understand what I thought I did years ago. Maybe my problem is that as much as I like Date and Darwen's points of view in that they make sense, I seem to mentally switch-off every time somebody criticizes SQL for its omissions which to me is like flogging a dead horse.)

regards,
pc Received on Sun Nov 28 2004 - 16:02:48 CET