Re: The Practical Benefits of the Relational Model

Paul Vernon <paul.vernon_at_ukk.ibmm.comm> wrote in message news:<ancq69$t0k$2_at_sp15at20.hursley.ibm.com>...
> Out of interest, how does D4 insert into this view?
>
> (A UNION B) MINUS (A INTERSECT B)
>
> It seems to me to be entirely arbitrary to insert into A rather than B, or
> vis-versa, and only slightly less arbitrary to not allow any inserts at
> all.

I would suggest an analogy between your equation

V = (A UNION B) MINUS (A INTERSECT B) and algebraic equation

Z = X + Y

In the second case if we know Z, we cant solve it unambiguously for X and Y values. In order to resolve ambiguity, we have to add a second equation.

Likewise, if we add a second view

W = A minus B

then the view set {V,W} is perfectly updateable.

For example, if we see a transaction

insert 1 into V;
insert 1 into W;
commit;

then it's definetely an insert into A. On the other hand,

insert 1 into V;
commit;  

is certainly an insert into B. Received on Sat Oct 05 2002 - 00:21:33 CEST