XML DTD and Schema

XML DTD

The purpose of a DTD is to define the structure of an XML document.

 It defines the structure with a list of legal elements:

<!DOCTYPE note
[
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>

XML Schema

XML-based alternative to DTD, called XML Schema:

<xs:element name="note">

<xs:complexType>
  <xs:sequence>
    <xs:element name="to" type="xs:string"/>
    <xs:element name="from" type="xs:string"/>
    <xs:element name="heading" type="xs:string"/>
    <xs:element name="body" type="xs:string"/>
  </xs:sequence>
</xs:complexType>

</xs:element>

XML Validation

XML Validation

• XML with correct syntax is "Well Formed" XML.
• XML validated against a DTD is "Valid" XML.

A "Well Formed" XML document has correct XML syntax.

The syntax rules were described in the previous chapters:

• XML documents must have a root element
• XML elements must have a closing tag
• XML tags are case sensitive
• XML elements must be properly nested
• XML attribute values must be quoted

<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
<to>Tom</to>
<from>Jerry</from>
<heading>Reminder</heading>
<body>Don't forget Birthday Party this weekend!</body>
</note>

Valid XML Documents

A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD):

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE note SYSTEM "Note.dtd">

<note>
<to>Tom</to>
<from>Jerry</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

The DOCTYPE declaration in the example above, is a reference to an external DTD file.