Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Sat, 02 Jul 2005 07:01:01 GMT
Message-ID: <NYqxe.135226$KN7.7252062@phobos.telenet-ops.be>

VC wrote:
> Hi,
>
> [...]
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:IVhxe.135039$l56.6861917_at_phobos.telenet-ops.be...
>

>>Jon Heggland wrote:
>>
>>>Anyway, there is one thing that is bugging me when I read articles about 
>>>object data models like Calvanese/Lenzerini and Van den Bussche / 
>>>Paredaens. If objects are just identity points, and the properties of 
>>>objects are other objects, where do actual data values enter into the 
>>>picture?
>>
>>Data values are special objects that have one or more representations 
>>associated with them by which they are identified.

>
> What's that supposed to mean ? 'Value' is just an element of a domain (data
> type), like, say, '1' is_a_member_of Integer. Are you proposing a new
> definition of 'value' ?

Not really, I'm restricting it slightly. I'm using the one that says that a value is something that (1) has one or more representations, i.e., can be encoded in memory and (2) is identified by that encoding in the sense that some equivalence relation over all possible representations is defined and each value corresponds to an equivalence class defined by it.

More formally, we postulate the set of all finite value representations VR (you can take the strings over some finite alphabet, if you want to be more specific), we postulate an equivalance relation EQ over VR, then we define the set of all values as the set of equivalence classes defined by EQ.

>>Like LOTs and NOLOTs in NIAM.

>
> I looked briefly at LOT/NOLOT. Did not see much of [formal] defonitions
> though. Could you please provide a reference or reproduce one ?

Lexical objects are representations of values. Which is slightly different from the definition above because there is equivalence relation defined.

> [It's not idle curiosity on my part (as was with the 'return' word). I am
> genuinely interested what the current formalism for the object/network/xml
> data model is.]

XML doesn't belong in that row, it is an entirely different beast.

• Jan Hidders

Received on Sat Jul 02 2005 - 02:01:01 CDT