An introduction to DTDs

• XML allows you to set conditions that must be enforced on an XML document.
• To define these conditions, you use a schema language to create a schema.
• Document Type Definition (DTD) is a schema language that's part of standard XML.

Rules for the products.xml document

• The document must contain one and only one Products element.
• The document can contain multiple Product elements.
• Each Product element must contain two elements named Description and Price.
• Each Product element must contain one attribute named Code that holds a string.
• The Description and Price elements can contain text data, but they can't contain child elements.

A DTD that implements the products.xml rules

<?xml version="1.0" encoding="UTF-8"?>
<!-- DTD for the products.xml file. -->
<!ELEMENT Products (Product*)>
<!ELEMENT Product (Description, Price)>
<!ATTLIST Product Code CDATA #REQUIRED>
<!ELEMENT Description (#PCDATA)>
<!ELEMENT Price (#PCDATA ) >

Coding a DTD

• You use the ELEMENT declaration in a DTD to define the names of the elements and the types of data they will contain.
• You use the ATTLIST declaration to define the names of the attributes and the types of data they will contain.
• By default, each child element must occur one time. To specify that a child element can occur:
• zero or one time, code a question mark after the name of a child element on the parent element declaration
• zero or more times, code an asterisk
• one or more times, code a plus sign
• In an XML document, you can use a DOCTYPE declaration to refer to a DTD that's stored in a DTD file.