Re: RM VERY STRONG SUGGESTION 4: TRANSITION CONSTRAINTS
Date: Sun, 5 Sep 2010 05:44:49 -0700 (PDT)
On Sep 5, 4:09 am, Erwin <e.sm..._at_myonline.be> wrote:
> > > You are free to think as you like. But my position (which is
> > > also Date's) allows me to use the CWA, and the CWA alone, to determine
> > > whether or not some tuple should or should not appear in some relvar.
> > The CWA has absolutely nothing to do with whether or not some tuple
> > should or should not appear in some relvar. The CWA applies to how
> > the tuples that don't appear in a relvar should be interpreted: false,
> > or unknown.- Tekst uit oorspronkelijk bericht niet weergeven -
> Introduction to Database Systems, 8ed, page 161:
> "In fact, the Closed World Assumption says that if an otherwise valid
> tuple -that is, one that conforms to the relvar heading- does not
> appear in the relvar body, then we can assume the corresponding
> proposition is false."
> But of course you will then argue that Date oversimplifies.
> Oversimplifies to 2VL, in this case. Because you obviously seem to
> insist that (WITHIN DBMS systems) 'unknown' is a needed logical
> construct distinct from 'false' and 'true'. And since the CWA is so
> foundational to the RM, in fact you are saying that Date doesn't
> understand the relational model. You are free to hold that belief.
You misunderstand. I am NOT saying that Date doesn't understand the relational model. Under the open world interpretation, when a tuple that can be in the database isn't, it is not automatically assumed that the corresponding proposition is false. Under the closed world interpretation, on the other hand, it /is/ automatically assumed that the corresponding proposition is false. In other words, if you don't know, then it ain't so. That still has absolutely nothing to do with whether or not some tuple should or should not appear in the database. Under both the open- and closed-world interpretations, only tuples that represent what is known to be true appear in the database; similarly, no tuple that represents what is known to be false appears in the database. The CWA says nothing at all about what is known to be true or what is known to be false; therefore, it has absolutely nothing to do with whether some tuple should or should not appear in the database. Received on Sun Sep 05 2010 - 07:44:49 CDT