Re: Declaring super types

Vadim Tropashko wrote:

>On Apr 19, 1:06 pm, r..._at_raampje.lan (Reinier Post) wrote:
>> ... "is a" does have a simple, useful and consistent
>> definition for the relational model: for relvars R, S, we can write
>>
>>   R is a S
>>
>> as a shorthand for:
>>
>>   + all attributes of S are attributes of R;
>>   + those attributes are a (possibly super)key of R, and
>>   + R projected on those attributes is always a subset of S.
>> ...
>
>Are you saying
>
>"R is a S"
>
>is eqivalent to
>
>"R join S = R"?

Hmmm ... that seems a nice shorthand for the first and third clause, but it doesn't imply the second one.

-- 
Reinier