Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Tue, 07 Jun 2005 21:10:32 +0200
Message-ID: <42a5f126$0$51821$e4fe514c@news.xs4all.nl>

Marshall Spight wrote:

>>Most of the time, in database context NULL denotes, as you point out,
>>the _absence_ of a value where you would expect one - still you are
>>freely using "NULL value" as a special kind of value.

>
> In fact, I don't think this is true. The SQL standard mostly takes
> the stand that NULL means "there is a value, but we don't know
> what it is." This is not the same thing as saying that there isn't
> a value.

True. Sure you know not everybody is happy about that (and the lovely 'mostly'), but I have to admit it talks about databases :-)

> If SQL's NULL was really the absense of a value, then 1+NULL would
> be 1. (The sum of a series of numbers that is one long is that
> number.) Instead it is NULL, which is consistent with "value exists
> but is unknown" and inconsistent with "the absense of a value."

Yep. Sharp. Did I get infected by some of Date's writings?

> In fact, it is my expectation that it would not be very hard to
> come up with a precise, useful, and not too hard to understand
> semantics for allowing the system to deal with "value absent."

I am less confident. BTW this reminds me of people saying it wouldn't be that tough to come up with a usable, relatively simple 'list' algebra.

> It doesn't appear to be possible to do this for "value not known"
> though; you get into this whole crappy 3VL space.
>
> You probably still would like to have user-defined special values.
Received on Tue Jun 07 2005 - 14:10:32 CDT