XML in a Nutshell, 3rd Edition

Uniqueness and Keys

Perhaps one of the most welcome features of schemas is the ability to express more sophisticated relationships between values in elements and attributes of a document. The limitations of the primitive index capability provided by the XML 1.0 ID and IDREF attributes became readily apparent as documents began to include multiple distinct types of element data with complex data keys. The two facilities for enforcing element uniqueness in schemas are the xs:unique and xs:key elements.

Forcing uniqueness

The xs:unique element enforces element and attribute value uniqueness for a specified set of elements in a schema document. This uniqueness constraint is constructed in two phases. First, the set of all of the elements to be evaluated is defined using a restricted XPath expression. Next, the precise element and attribute values that must be unique are defined.

To illustrate, let’s add logic to the address schema to prevent the same phone number from appearing multiple times within a given contacts element. To add this restriction, the element declaration for contacts includes a uniqueness constraint:

<xs:element name="contacts" type="addr:contactsType" minOccurs="0">
  <xs:unique name="phoneNums">
    <xs:selector xpath="addr:phone"/>
    <xs:field xpath="@addr:number"/>
  </xs:unique>
</xs:element>

Now, if a given contacts element contains two phone elements with the same value for their number attributes, the schema processor will generate an error.

This is the basic algorithm ...