问XML模式中属性值的唯一性

EN

<Vehicles>
    <Car id="0" name="Kia" />
    <Car id="1" name="Opel"/>

    <Plane id="0" name="Airbus"/>
    <Plane id="1" name="Boeing" />
</Vehicles>

<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:complexType name="VehicleType">
    <xs:attribute name="id" type="xs:int" />
    <xs:attribute name="name" type="xs:string" />
</xs:complexType>

<xs:element name="Vehicles">
    <xs:complexType>
        <xs:choice minOccurs="0" maxOccurs="unbounded">
            <xs:element name="Car" type="VehicleType" />
            <xs:element name="Plane" type="VehicleType" />
        </xs:choice>
    </xs:complexType>

    <xs:unique name="CarIdsMustBeUnique">
        <xs:selector xpath="Car" />
      <xs:field xpath="@id"/>
    </xs:unique>

    <xs:unique name="PlaneIdsMustBeUnique">
        <xs:selector xpath="Plane" />
      <xs:field xpath="@id"/>
    </xs:unique>

</xs:element>