From: Peter MATTHEWS (matthews_at_melbpc.org.au)
Date: Wed Jul 17 2002 - 08:54:36 CEST
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At 18:27 10-07-02 -0400, Richard Knapp wrote:
>One thing I'm having trouble in reading the model is differentiating
>between attributes and child entities. What exactly is a field?
[snip]
and
At 11:06 16-07-02 -0400, Richard Knapp wrote:
>On Tue, 16 Jul 2002 17:50:27 +1000, Peter MATTHEWS wrote:
>
>[snip]
> > What we are actually trying to get to is definitively finding out all the
> > data fields we need to consider when we design the XML part, and, just
> as ...
>
>... and Elements.
This flags up that I should have included some more definitions under
"Other Definitions", especially as XML's use of terms is different to
normal database use of terms. I have deliberately used the db term "field"
instead of the db term "attribute" because XML uses the term attribute in a
very specific way which is too restrictive at this stage of our
discussions. XML also uses the term "entity" in a specialised way quite
differently to normal English usage of the term and to db usage of the
term. We can revert to XML usage when we come to designing the XML itself.
While discussing the data model my usage has therefore corresponded to
normal usage of the terms "field" and "entity", so I propose adding their
definitions to the "Other Definitions" section as follows to make that clearer:
Entity
A real-world thing, event or concept that we want to record data about.
These are
represented by the squares in the diagram. Examples are Cave,
Instrument, Trip.
Field
A property of an Entity. In a database table, the fields are normally
represented by
the columns in the table. For example, "start date" is a field of the
entity "trip",
so in a table which listed trips, [Start Date] would be one of the
columns. Fields
are not being shown in the diagram, but will be listed out when we come
to discuss
them in detail in Task 6.3. In an XML file, any particular field would
be represented
either as an "element" or sub-element in its own right, or as an
"attribute" of another
element, depending on various considerations about that field.
Comments on these definitions? Are they acceptable at this stage?
Peter
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