Re: NULLs

On Dec 28, 8:31 am, Hugo Kornelis
<h..._at_perFact.REMOVETHIS.info.INVALID> wrote:
> On Wed, 26 Dec 2007 23:17:52 -0400, Bob Badour wrote:
> > Codd pointed out that a single NULL marker did not suffice and
> >suggested 2 markers. Date pointed out that one can apply the same
> >argument to 2 markers leading to an infinite progression once one heads
> >down that path, which suggests the path was never a productive one to
> >head down in the first place.
>
> Hi Bob,
>
> Unfortunately, both Codd and Date forgot that the NULL marker in (for
> instance) the age column should represent only that the age is not on
> file and not try to represent a reason for this as well.
>
> Codd's suggestions to use two markers (for "not applicable" and
> "unknown", IIRC) assumes that we want to store both the age of a person
> (if on file), and the reason why an age is not on file (if it isn't).
> That can of course be necessary - but in that case, we have two
> attributes that should be stored in two seperate columns.
>
> Attempting to store both the age and the reason why an age is unknown in
> a single column violates first normal form.
>
> Best, Hugo

This is Zaniolo's "missing information" definition of NULL which I agree doesn't suffer from the problems with associating NULL with a reason for the missing information (such as "exists but value unknown", "inapplicable" etc). Received on Fri Dec 28 2007 - 00:57:37 CET