Re: Entity and Identity
Date: Tue, 29 Sep 2009 21:27:49 +1000
> On Sep 29, 2:56 am, Clifford Heath <no.s..._at_please.net> wrote:
>> I don't support multiple inheritance for value types
> Why not?
Because Object Role modeling does not. Neither does it support unrestricted MI for entity types. The logical reasoning behind what is allowed and what is not is contained in Terry Halpin's PhD thesis, where he grounded ORM as a FOL by proving the mapping to KL. If you wish you find a definition that extends MI in these cases, you're welcome to attempt it yourself, but for me, my training does not fit me to attempt it.
>> and a value type cannot be derived from an entity type and vice versa.
> Please explain the difference.
A value type (aka lexical type) has a lexical form, or one which is commonly used to write its values. I can write a name or a phone number, but I cannot write a person or a telephone. In this sense, either a thing is, or is not, a value type; it cannot inherit both characteristics.
-- Clifford Heath, Data Constellation, http://dataconstellation.com Agile Information Management and DesignReceived on Tue Sep 29 2009 - 06:27:49 CDT