Functional Dependencies > Uniqueness Constraints

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.


And if that doesn't get you talking, maybe the Angry Cat will.

ph3ar the Angry Cat!

http://video.google.com/videoplay?docid=4479228649911767872


Marshall

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