Re: Multiple-Attribute Keys and 1NF
Date: Fri, 31 Aug 2007 07:21:04 -0400
Message-ID: <BQSBi.1702$z_5.1455_at_nlpi069.nbdc.sbc.com>
"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. >
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?
>> 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 - 13:21:04 CEST