Re: delete cascade

Brian Selzer wrote:
> "paul c" <toledobythesea_at_oohay.ac> wrote in message

>> 
>>Eg., the rva has an attribute that references the invoice# in the same 
>>relation that has the rva as an attribute.
>>

Well, if the view had been formed by a join of invoices and items and the only common attribute was invoice# and then some of the other attributes were grouped, invoice# would appear in only one of the headers.

That is different from saying invoice# in the rva is always redundant which would preclude a relation with invoice# as an attribute of both headers, for example, the tuple {invoice# x, rv {item#, invoice#}} where rv is empty. I'm not quite sure what use such a relation would have, but I don't see why one should take steps to prevent it if we can express it.

(By my definition, regardless of whether rv were empty, rv.invoice# could reference the "first" invoice# attribute.)

> ... Since a tuple
> is a value, not a variable, the rva containing the items would be assigned
> at the same instant as the containing tuple. This eliminates the need for a
> referential action. The thing I don't understand is why Bob suggested using
> a view. If composition rather than aggregation applies, then make the rva
> part of a base relation. A view would add unnecessary complication to the
> schema, leaving room for human error. I also am mystified that he mentioned
> cascading updates: from what he has said in earlier posts, keys can't be
> updated, and if keys can't be updated, there's no need for cascading
> updates.

On a logical level, I don't know what a "referential action" is. I am supposing it means the same as "reference". (I'm not sure I've ever understood why the foreign key kind of reference is not called implication as its definition seems to take the same form as logical implication.)

It is more Bob B's introduction of an rva that I am struggling with, so far I am guessing that it has to do with the ability to delete all items by deleting a single tuple.

p Received on Sun Apr 22 2007 - 22:00:30 CEST