Re: relationship in the database
Date: Thu, 26 Sep 2002 10:57:46 +0100
Message-ID: <amunp0$sai$1_at_sp15at20.hursley.ibm.com>
>But the consequence is then, that relational assignment is nothing
>more than a single tuple update. From a theoretical view, this might
>be satisfactory, but from a pragmatic view it is not.
Peter.
That is a messed up vision of what theory is, if you belive what you said
above.
>I believe that triggers (defined in the broad manner with e.g.
>FK-dependencies, not necessarily as stored procedures) are
>invaluable and a relational assignment must take the existence
>of triggers into consideration.
Agreed. Well sort of. What you need to ask is WHY are 'triggers' invaluable. You need to consider the fundamental issue(s) that the idea of 'tiggers' is/are attempting to address. In other words, try to approch 'triggers' from a theoretical angle :-)
>I will repeat myself (from another post): relational assignment
>sounds good at first sight, but not all practical problems can
>be solved via assignment as the transitional aspects are lost.
>In my mind, the consequence is that that what corresponds to
>an SQL UPDATE can not be supported via relational assignment
>and that some kind of update-operator must come into play.
>Those favouring relational assignment (and I actually am one of
>them)must define this operation in a formal way, taking into account
>in particular how entities are identified (when there are multiple
>candidate (or using Jan Hidders terminology: super-) keys). They
>must define the meaning of "new" tuples and tuples that disappear,
>and they must determine which triggers are called and when.
OK. In Date & Darewn's RM very strong suggestion #4: Transition constraints; they do introduce the concept of R' as the value of relvar R before an assigment to R.
I'll outline my answer to the 'problem' of 'SQL UPDATE not being supported
via relational assignment', in a response to a later post in this thread;
hopefully today.
>Purists may reject this on the grounds, that the purity of the
Again I question your concept of theory. I'd love to see someone of the
ilk of Steven Pinker write an analysis on the variety of philosophies of
knowledge. As in your earlier comment of 'do not wag the dog', Peter, you
betray a certain variety of the philosophy (just possibly shared by Jan,
but I'm defn not sure), that you do not, (in C J Date's words) believe
>model suffer, but the alternative is a model that does not reflect
>reality, and if you must choose between simplicity and reality,
>the choice should not be so difficult.
Regards
Paul Vernon
Business Intelligence, IBM Global Services
Received on Thu Sep 26 2002 - 11:57:46 CEST