# Re: NULLs: theoretical problems?

Date: Mon, 27 Aug 2007 07:20:39 -0700

Message-ID: <1188224439.501341.91410_at_19g2000hsx.googlegroups.com>

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