Re: 3vl 2vl and NULL

From: paul c <toledobythesea_at_oohay.ac>
Date: Mon, 05 Dec 2005 16:21:48 GMT
Message-ID: <wOZkf.47166$ki.43132_at_pd7tw2no>


David Cressey wrote:
> I've stayed away from the 3vl versus 2vl discussion over in the monster NULL
> thread. Mainly, I'm not sure exactly what Codd and Date have to say on the
> subject. I think that both of them are smarter than I am, and that they
> don't agree on the subject, and that they agree that they disagree. That
> pretty much leaves it up to us, doesn't it?
>
> Here's what I think: There are two Boolean constants: FALSE and TRUE.
> A boolean variable with a value must have one of these two values. I think
> that's the 2vl position, although I'm not sure.

Well, I'm glad for the sake of my eyes at least, that David C. has renamed this thread. For me, most of it is playing with words (no insult intended). That can be fun but it can also get us into trouble.

EG., "a ... variable ... might have no value" are dangerous words because we're on the verge of stepping out of the context we really mean, namely the value of an attribute in a tuple.

My basic attitude on the whole question goes back to where Codd started from, namely first-order predicate logic. As far as I know, FOL has no notion of nulls in the senses they are being talked about here. So, if one implants nulls into the RM, surely one must be prepared either to throw FOL out the window or invent a new flavour of FOL, which seems to me to be an undertaking that only a very few logic philosophers are capable of taking on.

I thought I'd mention this rather sticky point since nobody else seems to have brought it up and it seems fundamental to me that one must address it if one is going to entertain nulls.

Maybe I've got it wrong and Codd himself did try to do this sometime around 1979 but from what material is widely available, it seems he was prepared to discard FOL, here and there, for the sake of expediency. When I see people talking about special 'marks', I can't help but wonder if Codd wasn't subtly influenced by some of the hardware he would have been very familiar with and that had features like 'word marks' and 'field marks'.

paul c. Received on Mon Dec 05 2005 - 17:21:48 CET

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