Re: No exceptions?

Erwin wrote:
>> I keep five database textbooks in my office: Date, Elmasri/Navathe,
>> Garcia-Molina/Ullman/Widom, Ramakrishnan/Gehrke, and Riccardi. All of
>> them agree with me: a key is an irreducible superkey. Granted, two of
>> them say that "key" is short for "candidate key", but that does not
>> invalidate my position; one says that "candidate key" is what you call a
>> key when a relation has more than one; and the others don't use the term
>> at all, afaict. All agree that a proper superkey is not a key.
>>
>> Whose "contract" should I abide by? Yours, or theirs?

> 
> Check out Date.  I believe you'll find Bob's statements are in line
> with Date's.

Let me elaborate: Date, in his textbook (2003), defines "candidate key" and "superkey", to the effect that a candidate key is an irreducible superkey. He does not define "key", and does not use the unqualified term at all, afaict. Later, in TTM (2006), he (and Darwen, granted) defines "key" as synonym for "candidate key" (p. 26)---i.e. a key is irreducible---and uses the unqualified term almost exclusively.

Did he change his mind? Perhaps, or perhaps he just nailed down what the unqualified term "key" means. In any case it seems clear to me that when Date now uses the term "key", he means an irreducible superkey. Thus, to say that a key might be reducible has very little (if any) support from Date, might indeed contradict Date, and certainly contradicts several other textbooks.

Now, can anyone cite sources that treat "key" and "superkey" as synonyms? That is what the opposition is arguing, isn't it?

> And Date is the one who is closest to being the > "authentic source" for the relational model.

No argument there.

> If one agrees to abide by Date's definitions, then I believe your > statements indeed constitute redefinition.

I don't, as I explain above. Please explain your belief.

> If you don't agree to go by
> Date's definitions, then don't argue with anyone who does.

Why on earth not? Firstly, I don't contradict Date, and secondly, any hypothetical disagreement with Date does not disqualify me from the discussion.

Oh, and what about those who don't agree to Date's definitions of the extend operator, or of summarisation vs. aggregation? Should they shut up too?

-- 
Jon