Re: Multiple-Attribute Keys and 1NF

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.

>
> 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 - 12:37:36 CEST