# Re: NULLs: theoretical problems?

From: V.J. Kumar <vjkmail_at_gmail.com>
Date: Mon, 27 Aug 2007 07:20:39 -0700

Jan Hidders wrote:
> On 27 aug, 05:26, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

> > According to your own definition, 'def(x) : f(x)' evaluates to 'false'
> > whenever x is not defined. So f(x) in 'f(x) = x or not x' must be
> > 'false'.
>
> I really have no idea why you think that.

That's sloppy writing on my part: I should have written 'def(x) : f(x)'.

>
> > > > Likewise, 'def(x):(x or true)'
> >
> > > ... and is indeed equivalent with 'def(x):true'.
> >
> > This is interesting. Just a little while ago, you complimented Jon
> > for his understanding that 'select * from table where def(x): (x or
> > true)' is equivalent to 'select * from table where x is not null and
> > (x or true) '. Apparently, at the time you thought that 'def(x) :(x
> > or true) ' should evaluate to 'false'. It seems that now you think
> > that it should evaluate to 'true' !
>
> Not really. The formula 'def(x) : true' does not evaluate to true if x
> is undefined.

Ok. So it is at least consistent. Whenever we have def(x), any formula will evaluate to 'false'. So, as a result, we effectively lose the two tautologies I mentioned earlier. Please explain why is such loss is beneficial in comparison to the three-valued logic loss of one tautology.

>
> -- Jan Hidders
Received on Mon Aug 27 2007 - 16:20:39 CEST

Original text of this message