Re: Multiple-Attribute Keys and 1NF

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news: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?
>

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}?



>>>

>>> 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.