Re: Functional Dependencies > Uniqueness Constraints

erk wrote:
> Good topic!
>
> Marshall wrote:
> > Geeze, it's deader than a dotcom startup in here.
> >
> > Okay, fine. I propose that a good relational DBMS should
> > not have any special feature for enforcing uniqueness constraints.
> > Instead, it should be able to record and enforce functional
> > dependencies. (And of course there must be a rule that
> > says every base table must have at least one functional
> > dependency in which the union of the determinant set
> > and the dependent set equals the set of attributes. (This
> > restriction is sufficient to ensure every base table is a
> > relation; is it necessary?))
> >
> > Why? Because uniqueness constraints by themselves
> > don't capture enough meaning.
> >
> > Example:
> >
> > Relation R1(a,b) with FD {a} -> {b}
> > Relation R2(c,d) with FD {c} -> {d}
> >
> > Derived relation J = R1 join R2
> > (this join is a cross product)
> >
> > FDs in J:
> > {a} -> {b}
> > {c} -> {d}
> >
> > What if we had just recorded uniqueness constraints in R1
> > and R2 (on a and c respectively?) We can't express anything
> > derived from those uniqueness constraints in J, at least not
> > as uniqueness constaints anyway.
>
> Why not? Won't the candidate key in the derived relation consist of all
> the candidate key attributes in the "source" relations? For J, wouldn't
> it be {a, c}?

Well, uh, yes, that's right. But just knowing that {a, c} is unique isn't knowing the whole story, because we've "forgotten" that {a} -> {b} and {c} -> {d}. The system only records that {a, c} is unique, which is to say {a, c} -> {b, d}. Hence my original contention that uniqueness constraints don't capture enough meaning.

> In any event, even if we "just" specified uniqueness constraints on
> R1{a} and R2{c}, doesn't that imply the FDs you give above?

Yes for R1 and R2, but no for J.

> As for the P?=NP discussion earlier, I'm fairly ignorant, but in the
> specific case of a cartesian join with no overlapping attributes, isn't
> the only derivable CK just the union of the attributes of the CKs of
> the various "source" relations?

I think that's right.  

Marshall Received on Mon Sep 04 2006 - 20:04:45 CEST