XML, Web Services, and the Data Revolution

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XML, Web Services, and the Data Revolution

By Frank  P.  Coyle

Table of Contents
Appendix  A.   XML Language Basics

A DTD defines the structure of an XML document with a list of legal elements. It can be declared either within your XML document or as an external reference. If an XML document conforms to the rules set out by a DTD, the XML is said to be valid with respect to that DTD.

For example, consider the following XML document that contains an internal DTD that begins with " <!DOCTYPE " and ends with " ]> ".

<?xml version="1.0"?> <!DOCTYPE note [ <!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]> <note> <to>Bob </to> <from>Marilyn</from> <heading>Reminder</heading> <body>Don't forget to bring the cheese</body> </note>

The !DOCTYPE syntax in the second line defines this as a document of type note . The third line defines the note element as having four elements: to , from , heading , and body . The fourth line defines the to element as type #PCDATA, which means it may contain text. All the other elements are similarly defined.

A DTD may also be external to an XML source document. If so, the following syntax is used.

<!DOCTYPE root-element SYSTEM "filename">

Defining Attributes in DTDs

DTDs may also be used to specify attributes using the following form.

<!ATTLIST element-name attribute-name attribute-type default-value>

For example, the DTD

<!ATTLIST payment type CDATA "check">

establishes the validity of the XML

<payment type="check"/>

The attribute-type can have the values given in Table A-1.

As an example, consider the following attribute declaration as part of a DTD:

<!ELEMENT square EMPTY> <!ATTLIST square width CDATA "0">

Table A-1. Possible Values of the Attribute-Type in an XML DTD

Value Explanation
CDATA The value is character data
(en1en2..) The value must be one from an enumerated list
ID The value is a unique id
IDREF The value is the id of another element
IDREFS The value is a list of other ids
NMTOKEN The value is a valid XML name
NMTOKENS The value is a list of valid XML names
ENTITY The value is an entity
ENTITIES The value is a list of entities
NOTATION The value is the name of a notation
xml: The value is a predefined XML value

This maps to the following XML:

<square width="100"></square>

The element square is defined as an empty element with a width attribute of type CDATA . If no width attribute is given, the attribute has a default value of 0.

Default Attribute Value

The syntax for default attribute values is as follows :

<!ATTLIST element-name attribute-name attribute-type "default-value">

The following DTD example uses default values:

<!ATTLIST payment type CDATA "check">

Here is an example of XML that is conformant with this DTD:

<payment type="check"/>

Specifying a default value for an attribute means that the attribute will be assigned a value even if the author of the XML document does not supply one. Table A-2 shows possible default attribute values.

Table A-2. Default Attribute Values in an XML DTD

Value Explanation
value The attribute's default value
#DEFAULT The attribute's default value
#REQUIRED The attribute value must be included in the element
# IMPLIED The attribute does not have to be included
#FIXED The attribute value is fixed

Implied Attribute

The syntax for implied attribute values is as follows:

<!ATTLIST element-name attribute-name attribute-type #IMPLIED>

This is a DTD example using implied values:

<!ATTLIST contact fax CDATA #IMPLIED>

And the following XML conforms to this DTD:

<contact fax="214-555-6677"/>

Use an implied attribute in a DTD when you don't want to force the author to include an attribute and you don't have a default value.

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