Re: atomic

Quoth paul c:
> The relvars and keys example might be a little more interesting than
> mine. I'd say this is because D&D emphasize that an empty header can be
> a key. But if it is, then there would be only one tuple for a given
> relation that has an empty RVA, no other tuples [about that relation's
> keys].

Yes, but this is only a special case of the constraint that no key may be a proper subset of another key for the same relation. It is not some strange nullological phenomenon.

> I suppose
> if RVA's weren't allowed, a dbms that didn't support RVA's would have to
> presume the empty header as a key when no other set of attributes was
> specified.

I see no need for presumption. Without RVAs, an AMI would probably be in order, and by not associating any attributes with it, one would have an explicitly empty key, no?

-- 
Jon