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


Jon Heggland wrote:
> In article <1134049822.702505.277830@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
> boston103@xxxxxxxxxxx says...
> >
> > > I'll have to say that your scheme above seems to have all the flaws of
> > > Date's 2VL "special values" approach, *plus* extra complexity. YMMV.
> >
> > It's not my schema but rather Codd's.
>
> Not according to how I read the article we are arguing about.
>
> > That's how the SQL handles NULL (whatever NULL is assumed to mean).
>
> I am not convinced of that. Have you found the relevant passages in the
> standard?
>

In brief, the standard has this to say: there is an element/marker
(NULL) that extends each domain, and the rules for using the element in
those domains are such and such. The standard deals with the 3VL logic
by defining the truth tables for AND/OR/NOT and providing an additional
truth vale (UNKNOWN). Codd's treatment is similar except the vague
hint at uniformity. What I described, and you've called "[my] schema",
is similar to what the SQL standard does.

> > > I'd say that what they mean---the semantics---is defined to a great
> > > extend by how their operators work.
> >
> > What truth values, as well as various markers (I use the word marker
> > in Codd's sense, not in Date's) represent is in the
> > designer(s)/modeller(s) mind.
>
> I meant NULLs generally, not truth values.

Right, ok then, NULL behaviour is defined by the rules in the domains
other that the 3VL one.

>
> > > (And using numerals for both integers and floats is not the same thing.
> > > Tell me if you're interested. :)
> >
> > It's not entirely the same sure, due to the prominent role logic plays
> > in reasoning, but if one looks at the integers, rationals, 3VLs as
> > just algebraic structures with different rules of the game, the
> > analogy should be obvious.
>
> No, I think the way you used it was a bad analogy. From memory, you
> compared the use of NULL as (for example) zero for integers and as
> UNKNOWN for 3VL truth values with the use of 3 as integer and as
> rational. But the number 3 is the same value, the same concept,
> regardless of the declared type of the variable holding it.

That is not correct.

>Integer is a
> subdomain of rational.

That is not correct either. Mathematically speaking, integers form a
commutative ring and rationals are a field. Besides, you cannot say
that the rational 3 and the integer 3 are the same thing, because the
former is an equivalence class [canonical] representative over
integers which is surely not the same thing as the integer.

My point is, as it was before, that you can use the same label to
denote different things as long as you are careful with what you are
trying to say.

>Besides, '3' alone in programming languages
> typically signifies the integer; '3d' or '3f' are expressions of the
> same number in floating-point types. You can assign '3' to a FP
> variable, but that is because of implicit type conversion, not because
> the same symbol denotes both an integer and a FP number. (Of course, I'm
> assuming a strongly typed language.) None of this is analogous to the
> NULL question.
> --
> Jon

.

Relevant Pages

  • Re: So whats null then if its not nothing?
    ... the standard has this to say: ... > truth vale. ... And what is the relationship between NULL and UNKNOWN? ... > commutative ring and rationals are a field. ...
    (comp.databases.theory)
  • Re: Godels Theorem and Model Theory
    ... stuck imagining that the concept of truth is ... about truth of any kind (but only provability). ... logically necessary for the standard integers (or naturals) to be the ... The standard K-12 curriculum introduces ...
    (sci.logic)
  • Re: The incompleteness theorems, Sigma-1-completeness, induction, all that
    ... relevant until the SECOND incompleteness theorem, ... Obviously we already have truth in NATURAL language. ... Now it's true that Conmust be true in the standard model in order ... Obviously I meant there is no *cogent* objection. ...
    (sci.logic)
  • Re: Now you can see, my "crank" status
    ... It's like I'm fighting Republican ... > rational factors by focusing on the integers that make up rationals. ... and the truth will be apparent to all. ... Here you go, factor every non-prime integer between 1,000,000 and ...
    (sci.crypt)
  • Re: So whats null then if its not nothing?
    ... I meant NULLs generally, not truth values. ... > in reasoning, but if one looks at the integers, rationals, 3VLs as ... I think the way you used it was a bad analogy. ... assuming a strongly typed language.) ...
    (comp.databases.theory)