The XML
Document Type Definition is called the XML DTD. As the name implies,
it define the structure of an XML document. If you need to have a particular
structure for your XML document and want every user to create one based
on that structure then you have to create a DTD for that structure and
use it for validation of an XML document.
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An
XML document is considered a valid document if it follows the structure
defined in the DTD for that type of document.Know the syntax of the
DTD would help you a lot to create one. There are tools available that
would create the DTD for you if you have an XML document.
With this
method you can create a template of the DTD easily and then modify it
according to your requirement. In a DTD, the ELEMENT defines and element
and the child nodes are defined in the brackets. A sign like '+', '?',
and '*' are used after the name of the child nodes in the DTD.
These denote
the number of times the child node should exist in the XML document.
Attributes are defined using ATTLIST. The existence of an attribute
is controlled by keywords like, #REQUIRED, #IMPLIED, #DEFAULT, and #FIXED.
Many examples of DTD are available in the internet which can be used
for learning the concept.
