Re: Multiple-Attribute Keys and 1NF

On Aug 31, 2:28 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "JOG" <j..._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:

Just wanna emphasize the point that I'm not trying to sell anything here! I'm just exploring the idea (outloud).

>
> 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}}?

Yup I guess a natural join would work exactly like that. Unless you of course you used RENAME so:
(R AS r1) JOIN (R AS r2) = {not got the energy to enumerate the 9 propositions, entailing 30 pairs}
However I'd imagine that before joins one would often be UNGROUPing first.

>
>
>
> >> 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 - 15:51:27 CEST