Re: Basic question?What 's the key if there 's no FD(Functional Dependencies)?

vldm10 wrote:
> paul c wrote:

>> David Cressey wrote:
>>> "paul c" <toledobythesea_at_dbms.yuc> wrote in message
>>> news:a2c2h.242940$R63.209531_at_pd7urf1no...
>>>> vldm10 wrote:
>>>>> saturnlee_at_yahoo.com wrote:
>>>>>> What 's the key for it? ABC or nothing???
>>>>> ABC is not the key.
>>>>> Example: Let one partricular entity has A,B,C atributes
>>>>> and let these atributes take the following values:
>>>>>
>>>>>     A        B         C
>>>>> -----------------------------
>>>>>     2        4          6
>>>>>     8        4          6
>>>>>     2        4          6
>>>>>
>>>>> ( ABC  can be the key only in the trivial cases i.e if an entity has
>>>>> the atributes whose values never change)
>> ...
>>>> 2) I don't know why entities need to be mentioned, either, nor what a
>>>> non-"trivial" entity might possibly be.
>>>>
>>>> p
>>> I think he was referring to "trivial functional dependencies".  A key
>>> determines any subset of itself, trivially.  In current parlance,  "well,
>>> duh!"
>> I still don't get what entities have to do with FD's.  I thought Codd
>> came up with functional dependencies for relations, not entities and
>> that it is dangerous to mix those terms up, whatever we might think an
>> entity is, once we've made a relation to deal with it, we should suspend
>> the term as it can lead to all kinds of subjective confusions and just
>> talk about tuples or predicates (when people start talking about the
>> "real world", for me it's usually a clue that they are about to lapse
>> into mysticism!).
>>
>> p

No, I don't either. Only a mystic could.

> 1)
> Now if we put on left side of (*) ...

Stop right there. You haven't defined whatever kind of 'schema' or correspondence you are intending with the nine hyphens and the right arrow, so I don't see how you can start putting entities on one side of it. Just because you feel like it?

the entity from my example with
> its three states and try to apply given relation R(A,B,C) it doesn't
> work.

This is mixing up terms, equating states of entities with values of relations.

> Here we assume that ABC is the "natural" key.
>
> 2)
> If you try to apply just the last two states from my example into given
> relation R(A,B,C), that is case when values are not repeated,
> you will get something like:
>
> R: A B C
> ------------
> 8 4 6
> 2 4 6
> If you have millions of the rows in this relation, I don't know how
> you
> will determine what keys determine same entity (with different states).

I believe you are talking about bags, which RT says nothing about as far as I know.

>
> 3)
> I wrote about this earlier:
> Honda store received group of new Honda Civic, all of them with same
> attributes
> and we concluded that in this case "natural" key doesn't work.
>

Never had a Civic but all my Honda two-wheelers had unique serial numbers. None of them had all the same attributes but if they had, I would have made up my own serial numbers. I believe Codd called this the information principle. Without it, some applications are impossible, such as the one you are making up. I believe this is called a straw man.

p Received on Fri Nov 03 2006 - 15:12:19 CET