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   Re: [xml-dev] [schema] which assertions can't you express ?

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Philippe Poulard wrote:
> hi,
> An XML Schema is the expression of some assertions expected on an XML
> document class. 

That's an excellent definition, one of many possible definitions.

> Assertions on XML documents ensure that applications
> will process them without causing faults. 

If by "them" you mean XML documents, then that would not be true, as
there are many other factors - for example, did the schema cover all of
the required validations per the requirements (those that XML schema is
capable of covering)? And then there may be requirements that XML schema
cannot cover, some of which you discuss below.

> Expressing assertions with
> schemas ensure that applications developpers will spend most of their
> time in designing data process and few of their time in controlling them.

Again, it depends on the requirements and what is required beyond what
XML schema can handle and therefore may have to be enforced in
application code, database triggers, etc.
> Any schema technology is designed to cover numbers of assertions
> expressed. However, if think that the existing schema technologies (DTD,
> W3C XML Schema, Relax NG, Schematron) can't express some constraint types.

Speaking only for XML Schema, that is true.

> I have some ideas of what these constraint types might be, but I'd like
> to know, helped by your own experiences, which kind of useful assertions
> can't be expressed with these schema technologies (so frustrating).
> For example :
> -i'd like that the number of occurrences of an element is equal to a
> value of a given attribute (Schematron could do that)

Cannot be done with XML Schema.

> -i'd like that the value of a given attribute exists in my RDBMS

Cannot be done with XML Schema, but OASIS CAM can handle this.

Hope that helps--

Kind Regards,
Joe Chiusano
Booz Allen Hamilton
Strategy and Technology Consultants to the World

> -etc...
> Thanks in advance
> --
> Cordialement,
>             ///
>            (. .)
>   -----ooO--(_)--Ooo-----
> |   Philippe Poulard    |
>   -----------------------
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Kind Regards,
Joseph Chiusano
Booz Allen Hamilton


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