Re: Multiple-Attribute Keys and 1NF

From: Brian Selzer <brian_at_selzer-software.com>
Date: Fri, 31 Aug 2007 09:28:28 -0400
Message-ID: <1IUBi.610$7P7.428@newssvr19.news.prodigy.net>

"JOG" <jog_at_cs.nott.ac.uk> wrote in message news:1188556656.192653.305160_at_r23g2000prd.googlegroups.com...

> On Aug 31, 3:13 am, "Brian Selzer" <br..._at_selzer-software.com> wrote:

>>[snip]
>> "JOG" <j..._at_cs.nott.ac.uk> wrote in message
>> > Well, I have to contest again - you are no doubt referring to "rule
>> > 2:The guaranteed access rule", and that makes no reference to the term
>> > identity (...and that is what you asked me about.) Rule 2 is stating :
>> > "every individual value in the database must be logically addressable
>> > by specifying the name of the table, the name of the column and the
>> > primary key value of the containing row."
>>
>> Pardon me for being a stickler about this. I got this from dbdebunk:

>
> no worries - stickling is fine.
>

>>
>> "Each and every datum (atomic value) is guaranteed to be logically
>> accessible by resorting to a combination of table name, primary key value
>> and column name."

>
> Coupla things - Date and Darwen argue against the idea of atomicity,
> and they also complain about the use of 'primary key'. I also think
> Codds use of the term datum is incorrect. Throughout history data has
> required an attribute-value pair. The word is derived from the latin
> for 'statement of fact', its use in science always requires that the
> value is described. Its common sense really - if we don't know what a
> value means, well its just noise. Imagine the binary value 1000001.
> Ascii(1000001) makes it an A, Number1000001) makes it 65, etc.
>
> Either way, this doesn't matter as long as we know what each other
> mean.
>

>>
>> A datum is an /atomic/ value, not an individual value. Atomic--implying
>> that it cannot be separated into components.
>>
>> So having more than one value for a particular role violates the
>> guaranteed
>> access rule either way you look at it. If the column names aren't
>> unique,
>> then you can't access a particular datum by a column name. If a value is
>> a
>> collection of component values, then you can't access a particular datum
>> (component value), but only the collection in which it is contained.

>
> Well I've never suggested multiple values contained in a collection.
> But yes as I said, multiple roles does break the guaranteed access
> rule. My question is now (in the continuuing hunt for the theory
> behind 1NF)  is why on earth would that be a problem? I don't see any
> affect on the relational algebra.
>

How do you deal with join:

R {{A:4,A:5},{A:5},{A:5,A:6}}

Wouldn't R JOIN R =
{{A:4,A:5},{A:5},{A:5,A:6}, {A:4,A:5,A:6}}?

>>
>> But you're right that accessibility has nothing to do with identity. A
>> value can appear many times in many different tuples and in many
>> different
>> relations. Logical identity ensures that no matter how many times a
>> value
>> appears in a database, it always maps to the same individual in the
>> universe
>> of discourse.
>>
>> > Logically "addressable" - that's a very different kettle of fish to
>> > identity. In your original question did you mean to ask then: "What
>> > provides logical addressibality?" if one has two attributes playing
>> > the same role? I won't respond to that in advance, because I don't
>> > want to put words into your mouth. Regards, J.
>
> Received on Fri Aug 31 2007 - 08:28:28 CDT