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 17:59:09 GMT
Message-ID: <NBAxe.135734$rx6.7273595@phobos.telenet-ops.be>

Jon Heggland wrote:
> In article <NYqxe.135226$KN7.7252062_at_phobos.telenet-ops.be>,
> jan.hidders_at_REMOVETHIS.pandora.be says...
>

>>VC wrote:
>>
>>>"Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message 
>>>news:IVhxe.135039$l56.6861917_at_phobos.telenet-ops.be...
>>>
>>>>Data values are special objects that have one or more representations 
>>>>associated with them by which they are identified.

>
> [cut and paste]
>

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

>
> So a data value and a lexical object is not the same? A data value can
> have one or more representations, and a lexical object is such a
> representation?

Yes, under the definitions that I gave that is correct. Note that not everybody agrees with those, some would argue that 'data value' and 'representation' are synonymous.

>>>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.

>
> This sounds just like D&D's definition in TTM. But a lexical object is
> not a value, it is the representation of a value? What then is a lexical
> object *type*?

A set of values.

> Are there any restrictions on what types can be lexical object types?

No.

Jan Hidders

Received on Sat Jul 02 2005 - 12:59:09 CDT