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

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 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 Received on Thu Nov 02 2006 - 15:32:45 CET