Re: object algebra

> > Unless RDM changes its model, NULLs are forever an integral part of
> > RDM as Codd himself has realized.
>
> Codd was wrong.

Codd is right about NULL being an integral part of RDM. I will make a poor analogy. RDM says we are sending a Rover to Mars (unknown reality) to collect some flowers (data). NASA scientists (designers) put there heads together and determine a 3-cubicles container will be required (relation heading). When the Rover lands on Mars, it constructs a 3-cubicle box as pre-planned at design-time but only finds two flowers, the third cubicle remains empty (NULL). TDM doesn't have this problem because, it make a custom box.

> > implement www.xdb1.com/Example/Ex076.asp you will begin to see
>
> Unfortunately, that example illustrates precisely nothing. At least nothing
> recognizable as a problem I'd actually need to solve, though I'm not sure
> what problem is being referenced in it.

The first part is merely to represent what TDM/XDb1 does in a normalized, NULL-less manner. The second part is to generate the nearest common ancestor report. The example is artifically setup to emphasize the following point: some problems are easier to represent and code against using XDb1 because it is closer to relational algrebra than with RDM which is slightly further away from relational algebra.

The data to model is described by the following english-like sentences:

obeys isa relator. (Obey is used later to relate things in hierarchy)

god isa thing.
god equals god. (Here we create the god hierarchy) it is obeys. (Here we link the relator to the hierarchy, XDb1 specific)

force isa thing. (ArmedForces is a class) army isa force. (Other instances might be Navy, Marine, AirForce, etc)

church isa thing.
trinity isa church.

person isa thing.

john isa person.
mary isa person.
luke isa person.

age isa thing.
35 isa age.
john is 35.

weight isa thing.
130 isa weight.
mary is 130.

color isa thing.
red isa color.
luke is red.

dog isa thing.
fido isa dog.

computer isa thing.
laptop1 isa computer.

army obeys god. (Here we start relating things in the command hierarchy)
trinity obeys god.
john obeys army.
mary obeys army.
mary obeys trinity.

luke obeys trinity.

laptop1 obeys john.
laptop1 obeys mary.

fido obeys john.
fido obeys mary.
fido obeys luke.

For more details, see www.xdb1.com/Example/Ex076.asp Received on Fri Feb 27 2004 - 06:20:29 CET