Re: Little question for RDM theoristes
Date: 16 Jun 2006 06:37:37 -0700
> What you describe as "adding an attribute to a relvar" (and
> specifically : doing so in the very 'direct' sense that you imply) is
> *impossible* in the Date/Darwen RMD. It would involve an assignment of
> the form :
> R := R EXTEND someexpression AS someattributename.
O, No! TTM again.... :)
- Data model defines set of _types_ (Codd).
- _Type_ is set of _values_ (Codd, many well-known person and Date too).
3)D&D RMD prohibits assigment..... to _variables_(!) :) <<My question here - is D&D "RMD" model of _data_(values only) or it is model of _variables_(i.e. system which implements data model) too?>>
In other word if somebody describe a system (he can call it as "RMD" too:) ) where such assigment is not prohibited what will change in my previous post? Why do you use _variables_ when you try to prove me that header of realtion(what is _value_) is not data?
> This assignment cannot be compiled/executed on grounds of type
> incompatibility between the left- and righthand parts of the
This remark is not about values becouse L-part is variable.
> Adding an attribute to a relation variable implies changing the type of
> that relation variable. The fact of adding the attribute, by and in
> itself, does not involve the invocation of any relational operator.
> The fact of computing a new current value for this relvar, such that
> the value complies with the new definition of the relvar, might.
This remark is not about values - it is about variables.
> Compare it with changing the type of a program variable from "int" to
> "longint" : what arithmetic operator is invoked in doing so ?
There is no "int" and "logint" variables in arithmetic.I'm not speaking about implementation of data model (ever it is such abstract implementation like TTM's one). I'm speaking about data model itself.
:) I think it is a kind of differnece between theorists and experts. If I asked "is 100 and 20873495432523458374 values of different types" the theorist will answer "no" and expert answer "it depends on system". Received on Fri Jun 16 2006 - 15:37:37 CEST