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

From: Jon Heggland <>
Date: Wed, 30 Nov 2005 22:42:51 +0100
Message-ID: <>

In article <>, says...
> Please read the original article I've referenced for justification and
> the debate beween Codd and Date on the issue of nulls:

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. 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.

> 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.

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

> 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.

> > 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.

Received on Wed Nov 30 2005 - 22:42:51 CET

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