Re: 3vl 2vl and NULL

From: dawn <>
Date: 5 Dec 2005 19:18:16 -0800
Message-ID: <>

David Cressey wrote:
> "paul c" <> wrote in message
> news:wOZkf.47166$ki.43132_at_pd7tw2no...
> > David Cressey wrote:
> > > I've stayed away from the 3vl versus 2vl discussion over in the monster
> > > 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'.
> >
> I'm pretty much exhausted by this topic, and, as JOG said, it goes around in
> circles.
> But for your sake, I'll try and respond. (I'm still grateful to you for
> "it's easier to understand 600 tables than 100,000 lines of code").
> There is no reason whatsoever for FOL or any other order of logic to add the
> notion of nulls. It's simply outside the subject matter. It would make no
> more sense, and no less sense, to add the notion of nulls to the mathematics
> of integers.
> But when you're dealing with data systems, that's another story. If you
> deal with data systems, sooner or later you are going to have to deal with
> missing data. It's inherent in data systems. When you deal with missing
> data, you are either going to deal with it in a systematic way, or else
> you are going to deal with it in an unsystematic way.

I tried to visualize using a computer to address missing data in an unsystematic way. Can you expand on that?

> NULLS isn't the only systematic way of dealing with missing data, nor is it
> necessarily the best way. But it's definitely systematic, it's defnitely
> type independent, and it's definitely the way SQL chose. And, from my point
> of view, it's definitely good enough for a lot of practical work.

Sure, but after using power steering, I (and others) prefer not to turn back.

> And it's definitely more systematic and more type independent, than the
> practice, presented by some pickies in here of treating the empty string and
> the null as equivalent.

I gather you think that would over-simplify things. There certainly can be issues related to missing data however you model it. But I've worked with the 3VL of SQL and the 2VL of Pick and I find the latter much easier as an end-user as well as a developer. No matter what your requirements, you can handle them with this perhaps over-simplified approach or the surely over-complicated approach of a 3VL. The 2VL seems to give a bigger bang for the buck.

> Doing that blurs a distinction that is sometimes
> useful to make. And the fact that pickies are able to program their way
> around the absence of nulls is no proof that the notion is not a worthwhile
> one. You can program your way through a spaghetti bowl full of GOTOs as
> well. But that doesn't mean that you should.


> Notice I said "data systems" , not "database systems". While it's clear
> that Codd, in the 1970 paper, introduced the relational data model in
> connection with the problem of managing large shared databases, it's by no
> means clear that the data model is limited to that application.

In fact, there are data models backing web pages, printed pages, data exchange data, etc. Why is the RM largely connected to databases at this point when data modeling is everywhere and the RM is about modeling data?

> Indeed, if you'll let me quote from the beginning of the 1970 paper,
> "This paper is concerned with the application of elementary relation theory
> to systems which provide shared access to large banks of formatted data.
> Except for a paper by Childs [1], the principal application of relations to
> data systems has been to deductive question - answering systems. Levein and
> Maron [2] provide numerous references to work in this area."
> It's clear that some earlier work had been done with data systems other that
> databases, using the concept of mathematical relations.
> To my knowledge, Codd addressed the need for dealing with missing data as
> early as 1985, when he published the 12 rules.
> Here's rule 3:
> "Rule 3: Systematic Treatment of Null Values
> A field should be allowed to remain empty. This involves the support of a
> null value, which is distinct from an empty string or a number with a value
> of zero.

That is how the companies paying Codd to write this paper implemented the null. I don't think that was a coincidence.

> Of course, this can't apply to primary keys. In addition, most
> database implementations support the concept of a nun- null field constraint
> that prevents null values in a specific table column.
> "
> It's also clear that it's distinct from FALSE or TRUE, and is NOT another
> truth value. The wording "null value" above is unfortunate. The term
> "marker" is better. And I wouldn't get too bent out of shape by the word
> "marker". All we are dealing with, all the time, in data systems, are
> markers.

"Marker" works. I also like the term "signifier."

> I hope this makes my position clear: the NULL is a feature of DATA SYSTEMS
> and not a feature of any of the domains of the primitive dataypes,
> including the BOOLEAN datatype.

I think of it as a feature of a computer language.

> I'm close to the end of this topic now.
> Michael Preece is probably still
> thinking that SQL-relational people are "poor deluded fools".


> Maybe something will one day change his mind. Maybe not.

Every language has pros & cons. SQL's big pro is its wide use, adoption in client-server approaches such as odbc & jdbc, and by most database vendors. Maybe once those who have only worked with SQL start working with other tools (perhaps related to data in XML documents), there will be a move toward 2VLs end-to-end in software development. Then Mike can be in the mainstream and we won't need to change HIS mind. cheers! --dawn Received on Tue Dec 06 2005 - 04:18:16 CET

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