Re: Multiple-Attribute Keys and 1NF

On Aug 31, 12:34 pm, "Brian Selzer" <br..._at_selzer-software.com> wrote:
> "Brian Selzer" <br..._at_selzer-software.com> wrote in message
>
> news:BQSBi.1702$z_5.1455_at_nlpi069.nbdc.sbc.com...
>
>
>
>
>
> > "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?
>
> > 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?
>
> How would you deal with this:
>
> R {{A:3, B:3},
> {A:3,A:4,B:4}}
>
> UPDATE R WHERE A = 4 (A:=3)
>
> Supposing that A is the key,
>
> Do you allow {A:3,A:3,B:4}?
> Do you reject the update since {A:3, B:3} and {A:3, B:4}?
> Do you combine the B elements, {A:3, B:3, B:4}?
> Or does {A:3,B:4} supplant {A:3,B:3}?

If you are replacing (A,4) with (A,2) then

{ (A,3), (A,4), (B,4) }
=> { (A,3), (A,3), (B, 4) }
=> { (A,3), (B, 4) }

A is a key, so given (A,3) already exists in another proposition, the update is rejected.

>
>
>
> >>> 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 - 14:06:39 CEST