Re: The Practical Benefits of the Relational Model

"Peter Koch Larsen" <pkl_at_mailme.dk> wrote in message news:<3d996e20$0$52166$edfadb0f_at_dspool01.news.tele.dk>...

> While I have no doubt that Alphora has a much better view-update mechanism
> than SQL, I do fail to see any difference from SQL in your above claim. SQL
> supports view updates to, how ad hoc and bad these choices may be. What is
> needed is a more formal specification describing - in more detail - the view
> update policy used in D4. This would then be the base for arguing if the
> update is ad hoc and complete. I do not believe that this forum is
> appropriate for such a discussion though. What we might agree on then
> probably is that any view update mechanism involving set union or set
> difference would be ad hoc in the sense that alternative mechanisms could
> both succeed, but choosing among the different options must be based upon
> intuition and what feels right in the given situation.

Updateability is based on the "golden rule": "No statement is ever allowed to leave any relvar with a value that fails to satisfy its own relvar predicate." There _is_ a guiding principle underlying the semantics of each update operator, and this principle is consistently followed. This is in stark contrast to the updateability rules of SQL which are based on many factors including system implementation difficulty. Perhaps you do not agree with the principle guiding the semantics, but because the principle does exist, the semantics can hardly be called ad hoc.

Regards,

--
Nathan Allan