Dear Mitch, (01)
> On Fri, Jan 30, 2009 at 1:41 PM, Matthew West
> <dr.matthew.west@xxxxxxxxx> wrote:
> > Dear Mitch
> >
> >> - Any (relational) database schema describes a set of relations/an
> >> ontology and most are extremely multi-ary.
> >
> > [MW] Yes but these are mostly denormalisations to overcome the costs
> of
> > table joins.
>
> Are you saying that most of the table schemas you see in rdb's have
> large numbers of fields because they were purposefully re-enginneered
> that way (from collections of binary relations) in order to make
> access (and joins) more efficient? (02)
[MW] It depends whether the design was done properly or not. If it is done
properly then a (so-called) conceptual schema is developed that is neutral
to the processing that is done on the data. A conceptual schema is
considered "good" if it is highly normalised, i.e. the information is held
with no repetitions of the same data. It is then typically denormalised into
a physical (table) schema that takes account of the joins that are necessary
to do the most processing for the most important uses of the data. (03)
If it is done badly then the conceptual schema is skipped and the physical
design is not just denormalised, it is also constrained to simplify its
structure (e.g. many-to-many relationships restricted (simplified) to
one-to-many). Often this makes the systems difficult to use without entering
fudge or false data, but I guess most of you will have come across one of
those before now. (04)
Regards (05)
Matthew West
Information Junction
Tel: +44 560 302 3685
Mobile: +44 750 3385279
matthew.west@xxxxxxxxxxxxxxxxxxxxxxxxx
http://www.matthew-west.org.uk/ (06)
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>
> --
> Mitch Harris
>
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