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Introduction to XML For Web Developers
ID and IDREF  

ID represents a unique ID name for the attribute that identifies the element within the context of the document. IDs are much like internal links in plain HTML. For the most part, ID is used primarily by programs or scripting languages that process the document. The value for ID must be a valid XML name beginning with a letter and containing alphanumeric characters or the underscore character without any whitespace.

NOTE: ID is incompatible with the #FIXED keyword but usually appears in conjunction with the #REQUIRED keyword (we'll discuss these later). Of course, while ID is usually #REQUIRED, the reverse is definitely not true.

Also, take care that your ID values are _unique_ within a document!

<?xml version = "1.0"
         encoding="UTF-8"
         standalone = "yes"?>
<!DOCTYPE CONTACTS [
	<!ELEMENT CONTACTS ANY>
	<!ELEMENT CONTACT (NAME, EMAIL)>
	<!ELEMENT NAME (#PCDATA)>
	<!ELEMENT EMAIL (#PCDATA)>
	<!ATTLIST CONTACT CONTACT_NUM ID #REQUIRED>
	]>
<CONTACTS>

    <CONTACT CONTACT_NUM = "1">
    <NAME>Lok Siu</NAME>
    <EMAIL>siu@lok.com</EMAIL>
    </CONTACT>

    <CONTACT CONTACT_NUM = "2">
    <NAME>Joseph Misuraca</NAME>
    <EMAIL>joe@misuraca.com</EMAIL>
    </CONTACT>

</CONTACTS>

The IDREF type allows the value of one attribute to be an element elsewhere in the document provided that the value of the IDREF is the ID value of the referenced element.

<?xml version = "1.0"
         encoding="UTF-8"
         standalone = "yes"?>
<!DOCTYPE CONTACTS [
	<!ELEMENT CONTACTS ANY>
	<!ELEMENT CONTACT (NAME, EMAIL)>
	<!ELEMENT NAME (#PCDATA)>
	<!ELEMENT EMAIL (#PCDATA)>
	<!ATTLIST CONTACT CONTACT_NUM ID #REQUIRED>
	<!ATTLIST CONTACT MOTHER IDREF #IMPLIED>
	]>
<CONTACTS>

    <CONTACT CONTACT_NUM = "2">
    <NAME>Teri Mancuso</NAME>
    <EMAIL>teri@teri.com</EMAIL>
    </CONTACT>

    <CONTACT CONTACT_NUM = "1" MOTHER = "2">
    <NAME>Kristin Mancuso</NAME>
    <EMAIL>kristin@kristin.com</EMAIL>
    </CONTACT>

</CONTACTS>

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