XML XSD Schema - Enforce Unique Attribute Values in Schema

Question

Lets say I have a schema that defines the following XML:

<Values>     <Add Key="Key1">Value 1</Add>     <Add Key="Key2">Value 2</Add>     <Add Key="Key3">Value 3</Add>     <Add Key="Key4">Value 4</Add> </Values> 

I would like, at a schema level, to be able to enforce that the values for the Key attribute are unique, i.e. the example above is valid, but the following example would be invalid:

<Values>     <Add Key="Key1">Value 1</Add>     <Add Key="Key2">Value 2</Add>     <Add Key="Key2">Value 3</Add>     <Add Key="Key3">Value 4</Add> </Values> 

Notice that there are two Add elements with a Key of Key2

For reference here is the simple schema:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">     <xs:element name="Values">         <xs:complexType>             <xs:sequence>                 <xs:element name="Add" maxOccurs="unbounded">                     <xs:complexType>                         <xs:simpleContent>                             <xs:extension base="xs:string">                                 <xs:attribute name="Key" type="xs:token" use="required"/>                             </xs:extension>                         </xs:simpleContent>                     </xs:complexType>                 </xs:element>             </xs:sequence>         </xs:complexType>     </xs:element> </xs:schema> 

I am under the impression that this is not possible at a schema level, however I am all ears.

Answer 1

@BatteryBackupUnit has the right idea, but the syntax is more like:

<xs:element name="Values">   <xs:complexType>     <xs:sequence>       <xs:element ref="Add" maxOccurs="unbounded"/>     </xs:sequence>   </xs:complexType>   <xs:unique name="UniqueAddKey">     <xs:selector xpath="Add" />      <xs:field xpath="@Key" />    </xs:unique> </xs:element>