# Re: A different definition of MINUS, Part 3

Date: Sat, 20 Dec 2008 04:26:47 -0800 (PST)

Message-ID: <fb58533c-1924-4ea6-a123-ec6a323cf673_at_a26g2000prf.googlegroups.com>

[Snipped]

> So if the result of R JOIN S is {{A:3, B:6}, {A:3, B:7}}, then what are the

*> operands?
**> {{A:3, B:6}, {A:3, B:7}, {A:4, B:7}} and {{A:3, B:6}, {A:3, B:7}}?
**> {{A:3, B:6}, {A:3, B:7}} and {{A:3, B:6}, {A:3, B:7}, {A:4,B:6}}?
**> {{A:3, B:6}, {A:3, B:7}} and {{A:3, B:6}, {A:3, B:7}}?
**> Which?
**>
**> The whole notion that there is some algebraic solution to the "problem" is
**> ludicrous.
*

paul c and Vadim never claimed that algebric expression of the problem
is the solution. They are only trying to rely on a formalism which
has proven to be effective.

To qualify as *ludicrous* that motivation implies that you can offer a more credible alternative expression of the problem.

> > Why do we want to do that? Two reasons:

*> > 1. Database constraints are equations, and this generalization is a
**> > natural way to encompass them.
**>
**> That's an interesting take. I'm assuming that these equations can be
**> expressed in the algebra. Supposing that you have relation schemata R{A, B,
**> C} and S{A, D}. How would you express an interrelational constraint, such as
**> the inclusion dependency,
**>
**> R[A] IN S[A]
*

Date has clarified this aspect as subtyping.

> as an equation using the algebra? Or for that matter, how would you express

*> the functional dependency,
**>
**> AB --> C
**>
**> as an equation using the algebra?
*

Truth tables are easy to set up to validate FD in ra and FOPC.
Validating each fact can be easily formalized in ra.

> > 2. Information preservation. This one is easier to explain by the

*> > familiar linear system example. If there is not enough (linearly
**> > independent) equations, then there is a fundamental ambiguity of the
**> > inverse map that calculates input delta vector from the output.
*

I believe somehow that you are missing entirely the point paul and
vadim are trying to formulate. There is nothing wrong in expressing a
problem in a way that can allow people to communicate to exchange
ideas. That, ra is a proven effective way of expressing RL problems/
operation is a fact, not a matter of opinion.

To play devil's advocate (you'd be the devil), I would claim that ra does have its limits, but since you have not pointed any of them so far, I can only conclude that you missing the point.

Regards Received on Sat Dec 20 2008 - 13:26:47 CET