Re: Multiple-Attribute Keys and 1NF

On Aug 31, 12:21 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.
>
> What about restriction?
>
> R {{A:4, A:5, B:3},
> {A:3,A:4,B:4}}
>
> R WHERE A = 3?
> Do you return an empty relation, or {{A:3,A:4,B:4}}?
> If A = 3 is true, then A = 4 is also true, but shouldn't that be
> impossible?

Well in my own musings, the former. I viewed the WHERE clause in the light of set membership, so one asks whether the tuple contains the pair (A,3):
i.e. WHERE contains(A, 3) => { { (A,3), (A,4), (B,4) } }

      WHERE contains(A, 4) => { { (A,3), (A,4), (B,4) }, { (A,4), (A,5), (B,2) } }

But you could also ask for existence of tuples. i.e WHERE exists(A, 1) => {}
which is asking to return only propositions where there is only 1 pair featuring A as the attribute.

Or generally:
i.e. WHERE exists(Role, x) => { p &epsilon; R | &exist;x (Role, x) &epsilon; p }

>
> If A were a set, then you could write,
> R WHERE 3 IN A
>
> R WHERE A = 4 AND A = 5?
> Shouldn't A = 4 AND A = 5 always return false?
>
>
>
> >> 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 - 13:47:37 CEST