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> 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.
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