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$rx6.7273595_at_phobos.telenet-ops.be>
>
> [cut and paste]
>
>
> So a data value and a lexical object is not the same? A data value can
> have one or more representations, and a lexical object is such a
> representation?
>
> This sounds just like D&D's definition in TTM. But a lexical object is
> not a value, it is the representation of a value? What then is a lexical
> object *type*?
Date: Sat, 02 Jul 2005 17:59:09 GMT
Message-ID: <NBAxe.135734$rx6.7273595_at_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