"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote
On 2008-01-14 21:18:57 -0500, "Evan Keel" <evankeel (AT) sbcglobal (DOT) net> said: |
Always a physical issue. Never a theory issue.Agree?
Foreign keys are functional dependencies across two relations.
More specifically, let
R1(K1, A1, B1)
be a relation with attribute sets K1, A1 and B1 where K1 is R1's
primary key and B1 is a foreign key to the relation
where K2 is R2's primary key and A2 is the set of its remaining
Then the foreign key B1 represents the functional dependency
B1 --> A2,
which is the functional dependency across two relation I mentioned in
the first sentence.
Furthermore, through transitivity by the functional dependency K1 --
B1, the foreign key also represents the inter-relational functional
K1 --> A2.
Am I correct to say this?
Not reading the whole thread (big mistake on my part), but how about the
instance where a foreign key can be null?. No functional dependecy but