Re: Another Bidirectional Join Example

On Mar 30, 9:32 pm, "David Cressey" <cresse...@xxxxxxxxxxx> wrote:
Another example of the bidirectional self join is the "borders on"
relationship between two countries.

If France borders on Spain, it follows that Spain borders on France. If
Andorra does not border on Luxembourg, it follows that Luxembourg does not
border on Andorra.

Perhaps this one is easier to discuss than the soccer games example.

Minor related comments:

This "bidirectionality" is exactly the difference between edges in
a directed graph and an undirected graph.

A relation that exhibits this property is called "symmetric."
A relation R on AxA is symmetric if for all (a, b) in R,
(b, a) is in R.

I'm toying with how to express some of the queries
we've mentioned in my own notation. (Perhaps someone
can do the same in TTM?)


Marshall

.