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I would never try to model nulls by elements without contents. Either
(and preferably) model them by absent elements or - if you really need
the presence of the element for some reason - use a null-indicating
attribute (you may want to use xsi:nil).
Best regards
Michael - who has 7 years of experience arguing about mapping NULLs into
XML...
> -----Original Message-----
> From: Sudheshna Iyer [mailto:sudheshnaiyer@hotmail.com]
> Sent: Wednesday, November 30, 2005 2:41 AM
> To: davidc@nag.co.uk
> Cc: xml-dev@lists.xml.org
> Subject: Re: [xml-dev] Question on Allowing null values iin DTD
>
> David,
>
> Thank you for your reply.
>
> My requirement:
> We are building a common xml which is used by several vendors. Not all
the
> fields are
> valid for all the vendors. Particular vendor will fill whatever fields
> that
> are valid for
> him/her. So we should have elements which may have values or may not
have
> values.
> How can I achieve this by using DTD in xml?
>
> You posted:
> If you are wanting to model
> >a database entry that may be null, there are several ways that you
may
> >do this, by allowing the element to be empty
>
> MyQuestion:
>
> How can we do that? Empty values don't allow the values at all..
>
>
>
> >From: David Carlisle <davidc@nag.co.uk>
> >To: sudheshnaiyer@hotmail.com
> >CC: xml-dev@lists.xml.org
> >Subject: Re: [xml-dev] Question on Allowing null values iin DTD
> >Date: Tue, 29 Nov 2005 11:58:00 GMT
> >
> > > But I want to allow null values for element?
> >
> >XML does not have null values. It just has character content
(#PCDATA)
> >and nested element content (and attributes). If you are wanting to
model
> >a database entry that may be null, there are several ways that you
may
> >do this, by allowing the element to be empty, or by having an explict
> >child element, say <null/>, or having an explict attribute, it is
your
> >choice how to model this in teh XML, and you must then code your
> >application to understand this model. However you chose to model the
> >null value, the DTD declaration will reflect that model, you can't
> >declare "null" in a DTD.
> >
> > > I know that EMPTY element does not allow values. Then what is the
> >purpose of
> > > having EMPTY element in DTD?
> >
> >To declare elements that do not take eleemnt or character content,
such
> >as the XHTML img or br elements, both <img/> and <br/> are decared
> >EMPTY. Br often has no attributes either, img should have a src
> >attribute of course but it is still an empty element.
> >
> >David
> >
>
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