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  • From: Michael Kay <mike@s...>
  • To: John Cowan <johnwcowan@g...>
  • Date: Wed, 21 Dec 2016 08:58:04 +0000

On 21 Dec 2016, at 04:49, John Cowan <johnwcowan@g...> wrote:


On Tue, Dec 20, 2016 at 11:37 PM, Rick Jelliffe <rjelliffe@a...> wrote:

According to a neat site I found called Wikipedia, a 1975 ANSI expert group decided there are (at least) three major things we can call data models: physical data models (e.g. for JSON?), logical data models (e.g. for XML ?) and conceptual data models (e.g. RDF?).

Yes, but by "physical data model" they meant "the way tables are laid out in specific tracks and cylinders on disk."  Times change.


Actually, they meant anything you might want control over that affects performance, security, availability, etc without being visible at application level: for example, definitions of indexes, space allocation strategies, recovery journals, etc. I think that definition stands the test of time.

Exactly what ANSI SPARC meant by the "conceptual" schema has always been open to debate. The best definition I heard was "a schema written in a language that hasn't been invented yet". As soon as you define a concrete schema language with concrete semantics, it becomes a logical model rather than a conceptual one.

Michael Kay
Saxonica

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