Re: Multiple-Attribute Keys and 1NF
Date: Fri, 31 Aug 2007 14:42:37 -0400
Message-ID: <xiZBi.994$ZA5.983_at_nlpi068.nbdc.sbc.com>
"JOG" <jog_at_cs.nott.ac.uk> wrote in message news:1188568287.595590.272990_at_50g2000hsm.googlegroups.com...
> 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 there aren't any groups. I probably should have formatted R differently for clarity,
R {{A:4,A:5},
{A:5},
{A:5,A:6}}
Correct me if I'm wrong, but it seems to me that you're suggesting that multiple values for the same attribute can appear in the same "tuple." In that case it wouldn't matter if you renamed one of the operands of the join. The result would be the same.
>>
>>
>>
>> >> 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 - 20:42:37 CEST