Re: So what's null then if it's not nothing?
Date: 30 Nov 2005 07:47:52 -0800
Jon Heggland wrote:
> In article <1133288703.505685.293960_at_o13g2000cwo.googlegroups.com>,
> 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.
> "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
> 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
He uses the same symbol (NULL) both to talk about the unknown as an unknown value and to talk about the unknown as a logical constant as you noticed earlier yourself. It's confusing, but one can easily deduce from the context what exactly he means. In order to avoid confusion, one can use NULL to represent only an unknown value and UNKNOWN to represent the additional [to TRUE/FALSE] logical constant.
He does not, at least not in the article I've refered to.
Received on Wed Nov 30 2005 - 16:47:52 CET