Re: Functional Dependencies > Uniqueness Constraints

Jon Heggland wrote:
> Marshall wrote:
> > It is also true
> > that a system that supported FDs explicitly could express
> > everything that one that had only uniqueness could; in other
> > words, one concept is strictly more powerful than the other,
> > and I propose there is a language design principle that
> > says that in that situation, you should not include the weaker
> > concept.
>
> By that reasoning, FDs shouldn't be included if you have general
> database constraints.

Yes.

> And I want those.

I presume you mean, you want the general database constraints.

Given that an FD is just a particular kind of general database constraint, does that introduce any particular difficulty? Well, I guess it does actually, in that I expect it is undecidable whether a given constraint is an FD or not. But that doesn't mean we can't have a canonical expression of a constraint that is an FD, and be limited by the fact that unless we tell the system it's an FD (by using the canonical form) it won't know that it is. It seems to me this problem crops up in a variety of places, such as determining whether an operator is commutative based on its constraints.

Marshall Received on Wed Aug 30 2006 - 16:41:06 CEST