# Re: So what's null then if it's not nothing?

From: vc <boston103_at_hotmail.com>
Date: 30 Nov 2005 07:47:52 -0800

Jon Heggland wrote:
> boston103_at_hotmail.com says...
> >
> > Jon Heggland wrote:
> > > Well, like I said, Codd conflates the truth value "unknown" with NULL.
> > > I'm not sure that is a good idea.
> >
> > He does.
>
> Well, of course he agrees with himself. But who else does? How does he
> justify it? What are the consequences?

>
> > > I am curious: In Lukasiewicz's system, what do you get when you compare
> > > the unknown truth value to itself?
> >
> > Lukasiewicz's logic as far as I remember deals only with logical
> > connectives and "truth" values (0, 1, 2). Its truth table coincides
> > with Codd's 3VL, but I believe it has nothing to say about the value1
> > <comp> value2.
>
> http://en.wikipedia.org/wiki/Ternary_logic:
>
> "Note that any two statements with the same truth value are equivalent,
> even if the truth value is unknown."
>
> The truth table for equivalence (EQUALS, <->), 0=unknown, 1=true, 2
> =false:
>
> P Q P <-> Q
> 0 0 1
> 0 1 0
> 0 2 0
> 1 0 0
> 1 1 1
> 1 2 2
> 2 0 0
> 2 1 2
> 2 2 1
>

> > Codd just stipulates that the comparison results in
> > unknown if either operand is NULL. You are free to redefine ;)
>
> No, he also stipulates that the unknown truth value is the same as NULL,
> which means we are not able to represent the unknown truth value
> faithfully.

He does not, at least not in the article I've refered to.

> --
> Jon
Received on Wed Nov 30 2005 - 16:47:52 CET

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