Re: Foreign superkey support

Marshall wrote:
> paul c wrote:
>

>>Perhaps all I'm saying is that the basic relational algebra doesn't need
>>a foreign key concept which I admit is a deviation from the specific
>>question.

They are not. The algebra (or even better the calculus) is the language with which we describe the constraints.

> Constraints
> are a useful mechanism for ensuring integrity of variables, whereas the
> algebra is a way of constructing values from other values. (Including
> the values in variables.)

And including the boolean values of assertions.

  Note that the algebra is what you do queries
> with, but constraints are relevant only when doing DML: insert, update,
> delete.

Ironically, logical constraints can help with physical performance optimization even for data retrieval.

  (Although we may use the algebra to construct values that
> we then insert, for example.)
>
> It is quite interesting to consider the idea of constraints as
> descriptive entities for values, in addition to being prescriptive
> for variables. We can then consider propogation of these
> descriptions through the algebraic operations.

Yes, that is quite interesting especially with respect to view updatability. Received on Wed Aug 09 2006 - 02:52:15 CEST