Re: NULLs: theoretical problems?

JOG <jog_at_cs.nott.ac.uk> wrote in
news:1188264013.014106.58290_at_d55g2000hsg.googlegroups.com:

> On Aug 27, 10:30 pm, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

>> A natural question arises what is the def construct ? Is it a
>> classical logic extension ?
>
> PMFJI but I'm struggling to see why you believe there is a diversion
> from classical logic. How does def differ in your mind from the
> existential quantifier?
>

The existential quantifier does not really promise that something exists. The quantifier is just an 'OR' shorthand. So you need to have either strong/"existential" equality or a definedness operator to make this weird two-valued partial logic that Jan's so fond of work. None of this trickery is needed with the three-valued tools but they have their problems too !

> I am also unclear why one would ever favour 3VL in general.

I don't.

>When I ask
> for a subset from a set of propositions (via a select clause say),
> well, I need a DBMS to be able to fulfill that request. I can't ever
> imagine a situation where I'd want the system to be "unsure" of what
> goes into that subset. Surely by encoding accurately, not using
> metadata, etc that situation need never arise - isn't that
> instinctively more desirable?

The partial two vl and the three vl logics express roughly the same wrt. undefinedness so you can have a false sense of security with the former and be confused with the weak/strong equality, or have less verbosity but get confused with truth tables of the latter ! The problem is undefinedness, the logics are just a tool. Whether it's a good or bad tool is up to you to decide.

>
>>
>
> I personally don't see where the tautology is lost. For example:

It is not. I've misunderstood from the very beginning what Jan has been talking about hoping to hear something new and exciting but in vain, alas. Received on Thu Aug 30 2007 - 20:06:23 CEST