Re: Codd's Information Principle

On Oct 29, 7:40 pm, "Mr. Scott" <do_not_re..._at_noone.com> wrote:
>Suppose that a database embodies the
> following sentence,
> forall x forall y forall z Pxy \/ Qxz
> The database has two tables, let's call them P and Q, with predicates Pxy
> and Qxz respectively.

I don't understand what you are trying to say. The variable predicates and values determine the database proposition. If the predicates for variables P and Q are Pxy and Qxz then by definition the database proposition is (ie the database states that)

 (AND [for <x, y> in P] Pxy) AND
 (AND [for <x, y> typed by but not in P] ~Pxz) AND
 (AND [for <x, z> in Q] Qxz) AND
 (AND [for <x, z> typed by but not in Q] ~Qxz)

 (forall <x, y> in P Pxy) AND
 (forall <x, y> typed by but not in P ~Pxz) AND
 (forall <x, z> in Q Qxy) AND
 (forall <x, x> typed by but not in Q ~Qxz)

The dependencies are simply things that are true of the values that of P and Q will simultaneously hold. Their tuples are determined by their predicates and the way the world is. So there's no effect to adding them to the database proposition; they're always true.

> inclusion dependency from P[x] to Q[x].
>forall x forall y forall z Pxy iff Qxz

If P's xs must be in Q then
forall x forall y (Pxy -> exists z Qxz)
but that's not equivalent to what you wrote.

I don't know what you mean by "embodies". Implies? Is thus constrained?
Also "sentence" means "proposition" but I don't know what you mean by it.
And you seem to sometime use "database" when you mean "query result". I can't make much sense of your database comments. Your comments about the predicate for JOIN make sense.

philip Received on Sat Oct 31 2009 - 04:24:31 CET