Re: Relational query with path expressions

On Apr 3, 4:31 am, David BL <davi..._at_iinet.net.au> wrote:
> For example, given binary relation R(x,y), one could define a unary
> function f that maps a set of x values to a set of y values as follows
>
>     for all X, f(X) = {y | exists x in X, R(x,y)}
>
> It can be shown that f distributes over union and this "additive"
> property on sets is preserved over function composition.  In fact it
> is easy to show that composition of these unary functions is
> equivalent to a join + projection on the associated binary relations.

My interpretation of this is the following. You took binary named relation (Codd model operates named relations), and abstracted away named perspective. Then, you suggest that the join between binary relations is defined the same way as in algebra of binary relations. This allows you to express graph queries. Received on Mon Apr 06 2009 - 19:09:52 CEST