Re: NULLs: theoretical problems?

From: V.J. Kumar <vjkmail_at_gmail.com>
Date: Sat, 25 Aug 2007 02:09:26 +0200 (CEST)
Message-ID: <Xns9996CD0DE15AFvdghher@194.177.96.26>

Jan Hidders <hidders_at_gmail.com> wrote in news: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. 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 !

To sum up, when comparing the SQL three-valued logic with the DEFlogic,  we have a net loss of meaningfullness (see point 1) ! Apparently, the DEF logic behaves the same way as the SQL three-valued logic does in all the cases except (1). It brings us back to your original claim that the DEF logic is better that the SQL three-valued logic for handling undefined values. Am I missing something ?

Please comment.

>
> -- Jan Hidders
>
>
Received on Fri Aug 24 2007 - 19:09:26 CDT