From Oracle FAQ
A DTD (Document Type Definition) defines the elements or record structure of a XML document. A DTD allows your XML files to carry a description of its format with it.
Example DTD for a FAQ database:
<?xml version="1.0"?> <!ELEMENT faq-list (question+)> <!ELEMENT question (query, response*)> <!ELEMENT query (#PCDATA)> <!ELEMENT response (#PCDATA)>
- #PCDATA (parsed character data) means that the element contains data that can be parsed by a parser like HTML
- The + sign in the example above declares that the "QUESTION" element must occur one or more times inside the "FAQ-LIST" element.
- The * sign in the example above declares that the "QUERY" element can occur zero or more times inside the "QUESTION" element.
The W3C formulated a new standard, called XML Schemas that superceded DTD's. Schemas allow for more complex data types within your tags and better ways to constrain (validate) data within these tags.
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