# Re: NULLs: theoretical problems?

Date: Sat, 25 Aug 2007 11:18:02 -0000

Message-ID: <1188040682.225629.211180_at_q3g2000prf.googlegroups.com>

On 25 aug, 02:09, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

*> Jan Hidders <hidd..._at_gmail.com> wrote innews:1187998994.047351.228760_at_q4g2000prc.googlegroups.com:
**>
*

> > On 25 aug, 01:35, "V.J. Kumar" <vjkm..._at_gmail.com> wrote:

*> >> Are you saying that 'DEF t.a : (t.a = 5 OR TRUE)' evaluates to
**> >> 'false' ?
**>
**> > It evaluates to 'false' if t.a is undefined, and to 'true' if it is
**> > defined.
**>
**> >> Please give us the DEF operator interpretation rules. Without the
**> >> rules the discussion quickly becomes rather meaningless, really !
**>
**> > I've already done that twice. So for the third time: The formula "DEF
**> > c : f(c)" evaluates to true if c is defined and f(c) evaluates to
**> > true, and to false in all other cases.
**>
**> Very well. Now that we have the rules, let's consider some aspects of
**> the DEF logic that I've already mentioned but do not mind repeating my
**> words again:
**>
**> 1. The classical logic 'x or true=true' does not hold if x is undefined.
*

*> 2. The classical logic 'x or not x = true' does not hold if x is
**> undefined.
*

Also here, in the allowed formulas x cannot be undefined.

> Parenthetically, I find your complaint about the same

*> phenomenon in the SQL three-valued logic, well, mysterious taking into
**> account the fact that the DEF logic has the same defect !
*

It doesn't. In the allowed formulas it holds.

> Apparently,

*> the DEF logic behaves the same way as the SQL three-valued logic does in
**> all the cases except (1).
*

- Jan Hidders