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

Jon Heggland wrote:
> In article <1133365672.244152.260720@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> boston103@xxxxxxxxxxx says...
> >
> > 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 found no justification in the original article; that's why I asked! I
> have read the Codd/Date debate; it seems that Codd is confused about how
> 3VL works.

What specifically is he confused about with respect to 3VL ? His
truth tables are the same as Lukasiewicz's (for the logical operations
he talks in the article). His 3VL is not complete without the
implication, but perhaps he does not need it.

> But if we're just going to discuss like this, let's just say
> that I agree with Date and you with Codd, and leave it at that.

That's a non sequitur -- you cannot know whether I agree or disagree
with either. My point is that, if you cannot avoid using nulls for
whatever reason, Codd's 3VL with all its defects appears better that
other missing information handling alternatives. As an aside, if you
agree with Date, then NULL and 3VL should not even exist in your
toolset.

>
> > You are confusing the logical equivalence connective (<=>) with the
> > equality (=)predicate/relation. They are not the same thing,
> > naturally.
>
> Indeed? Seems silly to call it EQUALS, then. But please explain.

The 3VL equivalence is a logical operator (or connective) whose meaning
is defined by a truth table in the same way as it is for the 2VL
equivalence (sometimes called a biconditional). The equality operation
is a predicate/relation defined over a set of values.
I do not know why they call equivalence EQUALS.

>
> > 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.
>
> The problem is that NULL = NULL and thus UNKNOWN = UNKNOWN is not true,

There is no 'thus', the problem is that you are missing the context.
If NULL represents a 'missing value', the equality predicate over some
set of values extended so that it includes a special element NULL is
defined in such a way that NULL = NULL evaluates to UNKNOWN. Now, we
can consider a set consisting of three elements {TRUE, FALSE, UNKNOWN}.
It's a trivial observation that we can define equality over such set
where UNKNOWN = UNKNOWN evaluates to TRUE (UNKNOWN does not have any
special meaning, you could as well use QWERTY instead). Further, you
can use NULL instead of UNKNOWN, nothing really changes, you still
have a three element set {TRUE, FALSE, NULL} where you can define
equality so that NULL=NULL evaluates to TRUE. In other words, we
establish different rules for say {0,1,2,3, ..., NULL} and {FALSE,
TRUE, NULL}.


> whereas in a consistent logical system, UNNOWN = UNKNOWN is true, just
> like TRUE = TRUE and FALSE = FALSE is.

See above.

>
> > No, it is not. Similarly, Lukasiewicz uses 0,1,2, but you would not
> > claim that 2 is an integer in the context, would you ?
>
> I never claimed such a thing. I do claim that 0 = 0, 1 = 1 and 2 = 2,
> though.

See above.

>
> > > Do you know how Codd defines implication in his 3VL?
> >
> > He does not, at least not in the article I've refered to.
>
> Well, AND, OR and NOT (that he does define) are not sufficient to
> produce all functions in 3VL (unlike in 2VL), afaik. His theory has some
> holes.

That is correct.

> --
> Jon

.

Relevant Pages

  • Re: So whats null then if its not nothing?
    ... > the debate beween Codd and Date on the issue of nulls: ... I found no justification in the original article; ... have read the Codd/Date debate; it seems that Codd is confused about how ... > unknown value and to talk about the unknown as a logical constant as ...
    (comp.databases.theory)
  • Re: So whats null then if its not nothing?
    ... >>> whereas Codd claims that it is. ... > Now, if x is missing, 3VL will evaluate the expression to unknown. ... > that the answer should be true, and that tautology recognition ought to ... >> If the first zero denotes a logical constant, ...
    (comp.databases.theory)
  • Re: So whats null then if its not nothing?
    ... Jon Heggland wrote: ... > Well, like I said, Codd conflates the truth value "unknown" with NULL. ... >> original motivation. ...
    (comp.databases.theory)
  • Re: So whats null then if its not nothing?
    ... >> I found no justification in the original article; ... I think a proper 3VL with a proper UNKNOWN truth value is better. ... > equivalence. ... Because the result of the the evaluation of equivalence and equality ...
    (comp.databases.theory)
  • Re: So whats null then if its not nothing?
    ... UNKNOWN and MISSING to represent an address. ... > (person, opinion, veracity). ... > redefinition of the equality predicate. ...
    (comp.databases.theory)