XML Schema - Restrict element to enum or keyref

I am trying to write a schema, and I want to restrict the value or an element to either an enumerated list, or to a key reference. Is this possible? ie, im my schema I have

   <xsd:simpleType name="TypeEnum">
      <xsd:restriction base="xsd:string">
   开发者_开发百科      <xsd:enumeration value="uint8" />
         <xsd:enumeration value="uint16" />
         <xsd:enumeration value="uint32" />
         <xsd:enumeration value="uint64" />
      </xsd:restriction>
   </xsd:simpleType>

and in my XML i have

<root>
   <a>anEnum<a> <!-- This value should be restricted to either anEnum, anotherEnum or anything in TypeEnum -->
   <AdditionalTypes>
      <Enum Name="anEnum" />
      <Enum Name="anotherEnum" />
   </AdditionalTypes>
</root>

My question is how do I structure the schema element definition for <a>

Thanks in advance for any help.

---

Create another named simple type that allows the rest of the enumerated values ("anEnum" and "anotherEnum") just like you created the "TypeEnum" type. Then use <xsd:union> to combine these types and set <a> to use this combined type.

Code example

<xsd:simpleType name="TypeEnum">
    <xsd:restriction base="xsd:string">
        <xsd:enumeration value="uint8" />
        <xsd:enumeration value="uint16" />
        <xsd:enumeration value="uint32" />
        <xsd:enumeration value="uint64" />
    </xsd:restriction>
</xsd:simpleType>

<xsd:simpleType name="TypeAdditionalEnum">
    <xsd:restriction base="xsd:string">
        <xsd:enumeration value="anEnum" />
        <xsd:enumeration value="anotherEnum" />
    </xsd:restriction>
</xsd:simpleType>

<xsd:simpleType name="TypeUnionEnum">
    <xsd:union memberTypes="TypeAdditionalEnum TypeEnum" />
</xsd:simpleType>

<xsd:element name="a" type="TypeUnionEnum" />