paul c wrote:
> JOG wrote:
>
>> Change bothers me. Especially in database schema, and specifically when
>> we want to accomodate change in the cardinalities of the relationships
>> we are modelling. ...
>
>
> (I made a new subject because I didn't have anything particular to say
> about the original multiplicity question except that it refreshed
> questions about rva's that have been troubling me for a couple of months.)
>
> My questions are equally fuzzy, but I'll post them in case somebody can
> make a comment that gives me a better angle than the ones I've thought
> of so far.
>
> First, I have a presumption that two relations can express the same
> value in different ways, ie., via different descriptions or expressions.
> For example, most people are familiar with the axiomatic attitude
> that different writings of attribute/column order don't change anything
> material.
>
> The first question is, is there anything axiomatic about an sva
> (single-valued attributes) relation when compared to an rva relation
> that has the same attributes?
>
> For example, (using ttm-style braces and for convenience omitting type
> names), is a SUPP{S#} relation logically the same value as a SUPP{{S#}}
> relation that is (somehow) constrained to have only one tuple? (not to
> be confused with one that somehow allows multiple tuples).
>
> pc
They are not logically the same value; however, each is easily derived
from the other.
Received on Thu Apr 13 2006 - 15:38:52 CDT
