Re: attribute name conflicts

paul c wrote:

> Bob Badour wrote:
>

>> paul c wrote:
>>
>>> Bob Badour wrote:
>>>
>>>> paul c wrote:
>>>> ...
>>>
>>> ...
>>> Thanks, yes I did mis-type and meant it the way you put it.
>>>
>>> Regarding "capacity", I think I'd prefer for an app to use the three 
>>> different names: "volume", "weight" and "energy".  My main reason 
>>> (psychological) would be that I find it helpful to have as much 
>>> transparency as possible (as one might gather from reading my 
>>> frequent mis-typings) but a technical reason might be that then I 
>>> wouldn't need to deal with "exceptions" (I presume that an expression 
>>> like "A JOIN B" where A and B use the "capacity" attribute name for 
>>> different types would, in the purest sense, be considered not 
>>> well-formed, ie., it would not be strictly logical to give a result 
>>> such as "empty" or "false" for such an expresssion.)
>>
>>
>>
>> Whether such a join would cause an exception is a matter of applied 
>> psychology and not theory.

Actually, while the header of the result is correct, the body is incorrect. The body above should be empty because B does not contain any rows where d=2 and A does not contain any rows where d=3.

> Whereas if the header of B is {<c,t3>,<d,t2>}, with
>
> A:
> <c,t1> <d,t2>
> 1 2
>
> B:
> <c,t3> <d,t2>
> 1 3
>
> I'm guessing that you might not object to this result:
>
> A JOIN B:
> <d,t2>
> 2
> 3

I would object. The header of the result must have a 'c' attribute whose type is the most specific supertype of t1 and t3. Once again, the body would be empty.

> But I'm wondering if a psychological choice might also allow
>
> (A JOIN B) = Table_Dum, or even
>
> {A JOIN B} = Table_Dee,
>
> as long as a dbms was consistent in its choice.

Psychology cannot alter the correct header. Psychology can only choose between throwing an exception or returning an empty body with the correct header. Received on Thu Jun 28 2007 - 19:38:07 CEST