You can create a Unique contsrant within an XSD using the <xs:unique> element.
Examples:
Suppose you have the schema
<xs:element name="Directory">
<xs:complexType>
<xs:sequence>
<xs:element name="Person" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="Username" type="xs:string" use="required" />
<xs:attribute name="Age" type="xs:int" use="required" />
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
Then you could enforce a rule that forces usernames to be unique. You can do this using the Unique element.
<xs:element name="Directory">
<xs:complexType>
<xs:sequence>
<xs:element name="Person" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="Username" type="xs:string" use="required" />
<xs:attribute name="Age" type="xs:int" use="required" />
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:unique name="MyUserNameUniqueKey">
<xs:selector xpath="Person" />
<xs:field xpath="@Username" />
</xs:unique>
</xs:element>
The selector and field pair specify the data that must be unique, the XPath expressions are relative to the parent element (Directory).
Common Mistakes:
If the schema declares a targetnamespace then the XPath expressions must be fully qualified.
i.e.
<xs:schema xmlns:mstns="http://tempuri.org/XMLSchema1.xsd" elementFormDefault="qualified" targetNamespace="http://tempuri.org/XMLSchema1.xsd" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="Directory">
<xs:complexType>
<xs:sequence>
<xs:element name="Person" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="Username" type="xs:string" use="required" />
<xs:attribute name="Age" type="xs:int" use="required" />
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:unique name="MyUserNameUniqueKey">
<xs:selector xpath="mstns:Person" />
<xs:field xpath="@Username" />
</xs:unique>
</xs:element>
</xs:schema>
