Re: algebra equations for Reference and FD constraints
Date: Sun, 28 Dec 2008 02:03:17 -0500
"paul c" <toledobythesea_at_oohay.ac> wrote in message
> Brian Selzer wrote:
>> Second, this does not dispel the claim that there are some 'model'
>> concepts that can't be expressed with the algebra or calculus. In
>> particular, database updates cannot be expressed. To be sure, a value
>> that is to be assigned can, but the update itself--the actual
>> assignment--cannot. Nor should it.
> "update" is not a relational model concept nor is "assignment". They are
> both programming language concepts and are not necessarily present
> depending on the language, eg., some languages don't need assignment. Same
> goes for variables, aka pointers. Imputing any of these concepts to the
> relational model is making the same illogical mistake as criticizing the
> RM because of flaws in the SQL language. The mistake originates with the
> false assumption that a dbms implementation that may have been partly
> inspired by Codd's original model can somehow introduce or retro-fit
> concepts to that model, that he never ascribed to it. The mistake is
> mysticism at its finest. Whereas I would say that if one can't express a
> concept with either an algebra or calculus, then the concept is not a
> 'model' concept in the first place.
The Relational Model as Codd defined it involves what he called 'time-varying relations.' These are not the static relations that are the result of algebraic expressions--though their values at any particular point in time are. Your statement that neither 'update' nor 'assignment' are relational model concepts is patently absurd, for how can a relation vary with time unless there is some form of update or assignment involved.
<snipped irrelevant references to SQL> Received on Sun Dec 28 2008 - 08:03:17 CET