Re: Another Bidirectional Join Example
- From: "Cimode" <cimode@xxxxxxxxxxx>
- Date: 31 Mar 2007 04:10:02 -0700
On 31 mar, 07:02, "Marshall" <marshall.spi...@xxxxxxxxx> wrote:
On Mar 30, 9:32 pm, "David Cressey" <cresse...@xxxxxxxxxxx> wrote:Agreed.
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?)
I have always had a some trouble accepting the idea of a *de facto*
symetric relation without a formal mathematical demonstration.
Mathematical formalism dictates that in order to demonstrate the
specific characteristic of a relation, one has to initially assume
there are in 2 relations then establish that the relations are the
indeed the same for the property of symetry to be established. Quite
complex problem on which I have been biting my nails for a few years.
I hope this makes sense.
Marshall
.
- References:
- Another Bidirectional Join Example
- From: David Cressey
- Re: Another Bidirectional Join Example
- From: Marshall
- Another Bidirectional Join Example
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