XSLT, 2nd Edition

The Datatypes and How They Work

Three of the basic datatypes that are supported by XML Document Type Definitions (DTDs) and XML Schemas are ID, IDREF, and IDREFS. The ID and IDREF datatypes work according to two rules:

• Every attribute of datatype ID must be unique.

• Every value of datatype IDREF must match a value of an attribute of datatype ID somewhere in the document.

An attribute with a datatype of IDREFS contains one or more space-separated values, each of which must match a value of an ID elsewhere in the document. The IDREFS datatype is a list of IDREF values, just as its name implies.

Here is a simple DTD fragment that uses the ID and IDREF datatypes:

<?xml version="1.0"?>
<!-- parts-list1.xml -->
<!DOCTYPE parts-list [
<!ELEMENT parts-list       (component+, part+)>

<!ELEMENT component        (name, partref+)>
<!ATTLIST component        component-id ID #REQUIRED>

<!ELEMENT name             (#PCDATA)>

<!ELEMENT partref          EMPTY>
<!ATTLIST partref          refid IDREF #REQUIRED>

<!ELEMENT part             (name)>
<!ATTLIST part             part-id ID #REQUIRED>
]>

<parts-list>
...
</parts-list>

Here is the XML Schema definition of the same document type:

<?xml version="1.0" encoding="UTF-8"?>
<!-- parts-list.xsd --> <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xs:element name="parts-list"> <xs:complexType> <xs:sequence> <xs:element ref="component" minOccurs="1" maxOccurs="unbounded"/> <xs:element ref="part" minOccurs="1" maxOccurs="unbounded"/> </xs:sequence> </xs:complexType> </xs:element> <xs:element name="component"> <xs:complexType> ...