XSD Schema with unique id as attribute

Question

I want to write a XSD Schema for the following XML:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<siss-statusquery xmlns="http://www.example.de/test">
    <myhash id="1">DG5F6DFG13DFG5641DG5F6DFG13DFG56411337AS</myhash>
    <myhash id="2">123AWDFG13DFG5641DG5F6DFG13GFG56411337AS</myhash>
    <myhash id="3">DG5F6DFG13DFG5641DG5F6325DFG13DFG5641143</myhash>
</siss-statusquery>

Conditions:

• <myhash /> tag, minoccurs: 1 , maxoccurs: unbounded
• id attribute, type: unsignedInt , is unique , is required
• <myhash /> tag value pattern: [0-9A-Z]{40,40}

My attempt:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema attributeFormDefault="unqualified"
           elementFormDefault="qualified" 
           xmlns:xs="http://www.w3.org/2001/XMLSchema"
           targetNamespace="http://www.example.de/test" 
           xmlns:lhs="http://www.example.de/test">

  <xs:simpleType name="myhashType">
    <xs:restriction base="xs:string">
      <xs:pattern value="[0-9A-Z]{40,40}" />
    </xs:restriction>
  </xs:simpleType>

  <xs:element name="siss-statusquery">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="myhash"
                    maxOccurs="unbounded"
                    minOccurs="1">
          <xs:complexType>
            <xs:simpleContent>
              <xs:extension base="lhs:myhashType">
                <xs:attribute type="xs:unsignedInt" name="id" use="required" />
              </xs:extension>
            </xs:simpleContent>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

How can I constrict/restrict the id attribute as unique?

Answer 1

XML Schema has a type for globally unique IDs - xs:ID - but you can't use that here because values of this type have to be valid XML names (so in particular they can't start with a digit).

The way to apply more general uniqueness constraints is to use xs:unique :

<xs:element name="siss-statusquery">
    <xs:complexType>
       <!-- as before -->
    </xs:complexType>
    <xs:unique name="uniqueId">
        <xs:selector xpath="lhs:myhash" />
        <xs:field xpath="@id" />
    </xs:unique>
</xs:element>

The way to read this is that within the scope of the containing element, all the elements matched by the selector must have different values for their field . So in this case, within the siss-statusquery , all the myhash elements must have different id attributes.