Re: abnormal forms

From: paul c <toledobythesea_at_oohay.ac>
Date: Thu, 20 Apr 2006 21:43:12 GMT
Message-ID: <QfT1g.51142$7a.36958@pd7tw1no>

Bob Badour wrote:
> paul c wrote:
>

>> Bob Badour wrote:
>>
>>> ...
>>
>> If you have relation

>
>
> I assume by relation you mean relation variable.

Could be, once one is talking about an implementation. I didn't think I needed to go that far for a more elementary purpose.

> ...
>
> If you mean 'comparable' in the sense of the relational equality
> operator that compares two relations for equality, then the answer is
> yes. The result of the comparison is false.
>
>

>> and is it relationally comparable to
>>
>> 3. SP{S,P} with value
>>
>> S         P
>> =         -         (where the '=' underscore means S is a 'key')
>> 1         1
>> 2         1
>>
>> ?

>
>
> That would depend on the definition of the equality operator. Because
> the two relations have different types, the equality operator can either
> return false or cause a compile-time error.
> ...

I had in mind that there would be no such thing as an single valued-attribute except that all 'canonical' values would be single-element non-empty sets. I would have thought this could avoid typing errors.

 > ...

>> For sure, #3 is in a kind of canonical form as far as Codd was concerned.

>
>
> I disagree that #3 is a canonical form of either #1 or #2. Both #1 and
> #2 allow {{},1} but #3 does not.
> ...

Yes, I didn't want to make an example like that because to me it takes me toward negation and I haven't yet wrestled where that could lead. Same reason I tried to avoid talking about an 'equality operator'.

p Received on Thu Apr 20 2006 - 16:43:12 CDT