# Re: Constraints and Functional Dependencies

Date: Sat, 24 Feb 2007 15:58:19 GMT

Message-ID: <vgZDh.1112193$1T2.858868_at_pd7urf2no>

Marshall wrote:

> ...

*> R(a, b)*

> {a} -> {b} =def=

> forall R(a, b): forall R(a', b'): a = a' => b = b'

> ...

Just curious, regarding different constraints than keys,

- Does

R(a, b)

{a} -> {b} =def=

forall R(a, b): forall R(a', b'): a = a' AND b = b'

constrain R to a value that has at most one row?

2) Does

R(a, b)

{a} -> {b} =def=

forall R(a, b): forall R(a', b'): a = a' OR b = b'

constrain R to a value that has either zero rows or some number of rows that have the same value for one of a or b?

(If so, I suppose a and b could be seen as "alternate" keys, which one becomes key depends on the first two "insertions". Have I got that wrong?)

3) Does

R(a, b)

{a} -> {b} =def=

forall R(a, b): forall R(a', b'): NOT a = a' AND NOT b = b'

constrain R to a value has zero rows?

p Received on Sat Feb 24 2007 - 16:58:19 CET