Re: NULLs: theoretical problems?

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.

In the allowed formulas x cannot be undefined. So the logic doesn't say anything about whether it holds or not holds.

> 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).

There are other cases as well. All rules from 2VL logic apply in the allowed formulas so everywhere that 2VL differs from 3VL there def logic will also differ from 3VL.

• Jan Hidders