Re: Proposal: 6NF

On Fri, 06 Oct 2006 08:32:36 GMT, Brian Selzer wrote:

>
>"JOG" <jog_at_cs.nott.ac.uk> wrote in message
>news:1159970386.339044.87090_at_i42g2000cwa.googlegroups.com...
>> Brian Selzer wrote:
>>> "JOG" <jog_at_cs.nott.ac.uk> wrote in message
>>> news:1159954091.119164.155490_at_m73g2000cwd.googlegroups.com...
>>> > All of your points represent a wild goose chase in my eyes Brian. A
>>> > proposition with a NULL in it is no proposition at all. From a logical
>>> > perspective, case closed. A relation tuple with a NULL in it is no
>>> > relation tuple at all. From a mathematical perspective, case closed.
>>> > Trying to invoke the 'kludge perspective' is hardly going to convince a
>>> > theoretical newsgroup.
>>> >
>>>
>>> Is the empty set a value? Yes, it is. So why can't a null be?
>>
>> Because an empty set is a value and a NULL is not.
>>
>
>Why not?

Hi Brian,

Because relational databases supporting NULL *define* it as a marker denoting the absence of a value. Dawn actually makes a good point about context: in C for instance, NULL has a completely different meaning.

But since this is a discussion about relational databases, I'll assume the standard definition of NULL for relational databases unless you specifically state otherwise.

(snip)
>The empty set /indicates/ the absence of a value, yet it /is/ a value; a
>null /indicates/ the absence of a value, yet it /isn't/ a value? Why the
>double standard?

Wrong. The empty set *IS* a value. It's domain is the domain of sets. A set is a value that can hold zero, one or more values of a specified domain. The empty set happens to hold a zero number of values.

NULL is a marker that says "no value here". If NULL would appear in a column that is defined to use datatype "set" (not sure if any product supports it and not sure if I'd want it, but bear with me), then it would still be different from an empty set. Kinda like the difference between "the suitcase is empty" and "where's the &*(^^%#$ suitcase!"

Best, Hugo Received on Fri Oct 06 2006 - 23:35:05 CEST