Date and McGoveran comments on view updating 'problem'

"paul c" wrote in message

> Brian Selzer wrote:
>> "paul c" wrote in message
>>
> ... Also, you appear to be laboring under the
>> delusion that relational algebra somehow includes relational assignment.
>
> I feel no such weight on top of me because I know of no such thing as
> 'relational assignment', rather there is language assignment. My focus is
> algebraic as I'll try to explain later, but as far as that I would agree
> that the D&D A-algebra that I'm familiar with has no such thing as
> 'language assignment'. I don't think I ever said otherwise. I think
> Date's Assignment Principle is simply a test for whether a language's dbms
> respects the algebra it's premised on.

Perhaps I was reading too much into your post.

Bob Badour wrote:
....
> A more important question is: How would one write a constraint that
> deletes from SP ^ S affect SP but not S?
> ...

Using a typical relational algebra, my guess is that it would be
extremely lengthy, much longer than enumerating all the domains
involved. Eg., a disjunction with N-1 <OR> operations where N is number
of possible base headings multiplied by the number of possible
combinations of tuples in those relations, multiplied again by at least
three (since there are nominally three relations involved)! So, I don't
think it is a practical question as far as the RM as I know it is concerned.

Some language notation could be a shorthand for this, eg., S' = S might
mean that an S relvar can never be deleted from nor inserted to. One
might resort to a kind of catalog expression to establish some kind of
prototypical delete form where S' NOTEQUAL S MINUS X and X stands for
any possible relation, ie., any possible MINUS operand. But I'd say the
question is outside the realm of the algebra and therefore out of the
realm of the calculus (even though I know very little of the calculus).
Maybe if the RM admitted recursive structures, some algebra could
handle it and therefore some kind of terse algebraic constraint might be
possible.

Just guesses ...