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Re: Question on Allowing null values iin DTD

• To: davidc@n...
• Subject: Re: Question on Allowing null values iin DTD
• From: "Sudheshna Iyer" <sudheshnaiyer@h...>
• Date: Wed, 30 Nov 2005 05:41:12 -0500
• Bcc:
• Cc: xml-dev@l...
• In-reply-to: <200511291158.LAA09415@p...>

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@n...>
>To: sudheshnaiyer@h...
>CC: xml-dev@l...
>Subject: Re:  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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