Re: foreign key constraint versus referential integrity constraint

Marshall wrote:
...
> Consider relations A and B each with a single, common attribute.
> Natural join and inner union will behave much like intersection
> and union in this case. If the result type of the join isn't an
> intersection type, then we lose the property:
>
> A = A join (A union B)
>
> because the type of the attribute of the expression is different
> than the type of the attribute of A.
> ...

Marshall, I probably am diverging from your purpose but let me ask if that property is important because without it you don't have a relational lattice or is it important because without it some practical use is lost?

(I may be far out in left field as far as mathematicians and type theorists are concerned, but my simple interpretation of the Information Principle is that the type of A{c} JOIN B{c} is {c}, whether it is a union or intersection doesn't need to be asked.) Received on Tue Oct 27 2009 - 17:33:26 CET