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

.

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