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

Jon Heggland wrote:
> In article <1133280992.421387.165640_at_g44g2000cwa.googlegroups.com>,
> boston103_at_hotmail.com says...
> >
> > > Indeed? Is NULL an inherent part of 3VL? Reference? I don't think Codd
> > > invented 3VL ...
> >
> > In the Codd style 3vl it is. See the article I mentioned earlier. He
> > uses w (omega) to denote NULL and his NULL is "value at present
> > unknown".
>
> Well, like I said, Codd conflates the truth value "unknown" with NULL.
> I'm not sure that is a good idea.

He does.

> In any case, I'd prefer that you say
> "according to Codd's 3vl rules" instead of "according to /the/ 3vl
> rules" (my emphasis). But you may consider that splitting hairs.

You are right. Please substitute according to Codd's 3vl rules" for "according to /the/ 3vl rules" everywhere in my messages ;)

>
> > He did not invent the 3VL, there had been Lukasiewicz before.
>
> 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. Codd just stipulates that the comparison results in unknown if either operand is NULL. You are free to redefine ;)

>
> > > As for Codd's article, I think it is a bad idea in itself to use 3VL.
> >
> > I fully agree that nulls should be avoided as much as possible,
> > however, if one uses them one should understand their behavior and the
> > original motivation (by Codd).
>
> Of course. But I thought we were discussing whether Codd's nulls and
> rules were well designed or not. It is possible to envision
> alternatives.

There has been quite a few alternatives offered including Codd's own 4VL. However, they appear worse than the arguably not quite satisfactory Codd's original 3VL.

> --
> Jon
Received on Tue Nov 29 2005 - 19:25:03 CET