DTD Attribute Types - ID
The attribute type of ID is used specifically to identify elements. Because of this, no two elements can contain the same value for attributes of type ID. Also, you can only give an element one attribute of type ID. The value that is assigned to an attribute of type ID must be a valid XML name.
Syntax:
<!ATTLIST element_name
attribute_name ID default_value>
Example:
<!ATTLIST mountain
mountain_id ID #REQUIRED>
Valid XML - The following XML document would be valid, as it conforms to the above DTD:
<mountains>
<mountain mountain_id="m10001">
<name>Mount Cook</name>
</mountain>
<mountain mountain_id="m10002">
<name>Cradle Mountain</name>
</mountain>
</mountains>
Invalid XML - The following XML document would be invalid because the value of the "mountain_id" attribute is the same for both elements:
<mountains>
<mountain mountain_id="m10001">
<name>Mount Cook</name>
</mountain>
<mountain mountain_id="m10001">
<name>Cradle Mountain</name>
</mountain>
</mountains>
