Re: So what's null then if it's not nothing?
- From: Jon Heggland <heggland@xxxxxxxxxxx>
- Date: Wed, 30 Nov 2005 12:20:47 +0100
In article <1133288703.505685.293960@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
boston103@xxxxxxxxxxx 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. This is a blunder analogous to saying that the empty string
is NULL.
Do you know how Codd defines implication in his 3VL?
--
Jon
.
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