XSchema Spec Section 2.3, Draft 1

Ron Bourret rbourret at dvs1.informatik.tu-darmstadt.de
Mon Jun 15 15:54:31 BST 1998

Simon St. Laurent wrote:
> John Cowan wrote:
> >That was my original design.  Ron thought it was clearer to allow
> >an explicit PCData element equivalent to an empty Mixed element,
> >and I acquiesced.
> For now, I'll leave it.  Ron's away for a couple of days, and we can discuss 
> it when he gets back.

I'm back.  The caves in France found me unpalatable and spat me back to the 

Many DTDs have lots of PCDATA-only elements, so I included the PCData element as 
a clear, convenient shorthand.  That is, I found:

<ElementDecl id="foo">

to be clearer than:

<ElementDecl id="foo">

This is particularly true when you are transmitting non-document data such as 
corporate or scientific data.  For example, a simplistic sales order DTD might 

<!ELEMENT SalesOrder (Number, CustomerNumber, Date, Line+)>
<!ELEMENT CustomerNumber (#PCDATA)>
<!ELEMENT Line (PartNumber, Quantity)>
<!ELEMENT PartNumber (#PCDATA)>
<!ELEMENT Quantity (#PCDATA)>

Note that everything resolves very quickly to PCDATA -- it's just text, numbers, 
and dates and contains no embedded markup such as bolds or jumps or 
what-have-you.  There is nothing "mixed" about it.  The PCData element, while 
not strictly necessary, allows you to get this notion across quite easily.

(Note also that I considered a Scalar element instead of a PCData element.  With 
a Type attribute -- initially allowing PCDATA only -- we could expand into 
typing in the future, which the database community really needs.)

By the way, I like the fixed frequency of ZeroOrMore on Mixed.

-- Ron Bourret

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