Re: Demo: Modelling Cost of Travel Paths Between Towns

> I thought YOU were the Original poster, at least that's how it looks

The OP was in OT titled "Representing a Graph in an RDB...again...(BUT DIFFER...". For some reason, I failed to post there.

> > CREATE2 (a.goto=b).weekday = +monday;
> > The + indicates that if monday does not already exist as an instance
> > of the verb (weekday), go ahead and create it.
>
> How does the language distinquish between the operation or changing the
> value to +1 from the operation of adding a new instance with the value 1.

By enclosing it in quotes (ie CREATE2 (a.goto=b).weekday = +'+monday';). The portion after the optional + is the string that will name the new instance. The prior command creates a weekday named by the string '+monday' if it does not already exist. Currently UPDATE has not been implemented but it may look like this UPDATE2 john.age=35 to 54; or UPDATE2 john.favorite-word="to" to "too";

> Matching that model in a Relational data model is easy enough.
> (Joe Celko already posted a SQL model.)

This problem didn't not have highly variable data and relationships or require a higher degree of normalization. This problem falls within RM's scope. Try adding a different property to each city or name two cites the same. RM solution will require some changes to schema and code already written against. XDb2 solution will be minimally affected. Script below shows how:

// Add state property to existing town a CREATE2 *state.cls = thing;
CREATE2 a.state = +texas;

// Add population to existing town b
CREATE2 *population.cls = thing;
CREATE2 b.population = +10000;

// Add a new town a, but in different state CREATE2 *a.cls = town;
CREATE2 a.state = +florida;

In fact, in the original XDb2 solution, two towns could possibly have the same name. To distinguish them would require specifying which towns it goes to. For example suppose we change town c's name to string 'a' (via GUI). One can still find the original town a (top-left position) because only it goes to town b and what used to be town c (now named 'a') using the following query:

SELECT2 %.cls=town & %.goto=b & %.goto=(%.cls=town & %.name='a');

In RM's current solution, symbol and town name are the same thing resulting in inflexiblity in above case. In XDb2's solutions a symbol or string names a town which is a different thing than the symbol or string. RM has its own advantages which I did not list here. Received on Fri Nov 12 2004 - 05:47:34 CET