Foreign keys

#**151**

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote

Quote:

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining
attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

#**152**

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote

Quote:

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining
attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

#**153**

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote

Quote:

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining
attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

#**154**

Evan Keel wrote:

Quote:

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote in message
news:2008011502240916807-kirakun (AT) earthlinknet (DOT) ..

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining

attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

The legitimacy is predicated on the legitimacy of NULL in the first place.

#**155**

Evan Keel wrote:

Quote:

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote in message
news:2008011502240916807-kirakun (AT) earthlinknet (DOT) ..

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining

attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

The legitimacy is predicated on the legitimacy of NULL in the first place.

#**156**

Evan Keel wrote:

Quote:

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote in message
news:2008011502240916807-kirakun (AT) earthlinknet (DOT) ..

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining

attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

The legitimacy is predicated on the legitimacy of NULL in the first place.

#**157**

Evan Keel wrote:

Quote:

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote in message
news:2008011502240916807-kirakun (AT) earthlinknet (DOT) ..

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining

attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

The legitimacy is predicated on the legitimacy of NULL in the first place.

#**158**

Evan Keel wrote:

Quote:

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote in message
news:2008011502240916807-kirakun (AT) earthlinknet (DOT) ..

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining

attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

The legitimacy is predicated on the legitimacy of NULL in the first place.

#**159**

Evan Keel wrote:

Quote:

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote in message
news:2008011502240916807-kirakun (AT) earthlinknet (DOT) ..

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining

attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

The legitimacy is predicated on the legitimacy of NULL in the first place.

#**160**

Evan Keel wrote:

Quote:

"Kira Yamato" <kirakun (AT) earthlink (DOT) net> wrote in message
news:2008011502240916807-kirakun (AT) earthlinknet (DOT) ..

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
R2(K2, A2)
where K2 is R2's primary key and A2 is the set of its remaining

attributes.

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
dependency
K1 --> A2.

Am I correct to say this?

--

-kira

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
legit.

Evan

The legitimacy is predicated on the legitimacy of NULL in the first place.