Re: So what's null then if it's not nothing?
Date: 30 Nov 2005 07:47:52 -0800
Message-ID: <1133365672.244152.260720_at_z14g2000cwz.googlegroups.com>
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?
Please read the original article I've referenced for justification and the debate beween Codd and Date on the issue of nulls: http://www.dbdebunk.com/page/page/1706814.htm.
>
> > > 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.
>This is a blunder analogous to saying that the empty string
> is NULL.
>
> Do you know how Codd defines implication in his 3VL?
He does not, at least not in the article I've refered to.
> --
> Jon
Received on Wed Nov 30 2005 - 16:47:52 CET