XML-Schema: how to set enumerations to one appearance per document

问题

I want to add a name-attribute to all my elements. To make sure that only some attribute values are allowed I created a restriction-pattern. Is there a way to make sure, that each enumeration value is used exactly once per document?

simpleStyle:

<xsd:simpleType name="myname">
    <xsd:restriction base="xsd:string">
        <xsd:enumeration value="1"/>
        <xsd:enumeration value="2"/>
    </xsd:restriction>
</xsd:simpleType>

---

XML

<element  name="1"/>
<element  name="2"/>
<element  name="1"/>

The last one should not be allowed.

Is that even possible?

回答1:

There is a similar concept called identity constraints, which may suit your purposes. You want to use xs:unique, which is a constraint on the parent element, not a property of an enumeration. Something like:

<xs:unique name="myconstraint">
 <xs:selector xpath=".//*"/>
 <xs:field xpath="@name"/>
</xs:unique>

The selector says which child elements the constraint applies to (all descendants in this example), and the field indicates which part must be unique.

回答2:

As correctly suggested by @xan and by this guide, you could change your code this way:

1. Define the myname enumeration.
   This will allow only the usage of 1 or 2 values.

   <!-- The Enumeration definition (allowing only values 1 or 2) -->
   <xsd:simpleType name="myname">
       <xsd:restriction base="xsd:string">
           <xsd:enumeration value="1"/>
           <xsd:enumeration value="2"/>
       </xsd:restriction>
   </xsd:simpleType>
   

2. Define the element structure.
   This will define the structure for the <element name="..." /> node.

   <!-- the element definition (defines the <element /> structure) -->
   <xs:complexType name="elementType">
       <xs:attribute name="name" type="myname" use="required"/>
   </xs:complexType>
   

3. Univque usage.
   Define the element tag usage requiring an univoque usage of the name attribute.

   <xs:element name="myelements">
   
       <!-- The <element/> tag usage -->
       <xs:complexType>
           <xs:sequence maxOccurs="unbounded">
               <xs:element name="element" type="myname"/>
           </xs:sequence>
       </xs:complexType>
   
       <!-- The univocity constraint -->
       <xs:unique name="nameUniqueConstraint">
           <xs:selector xpath="element"/>
           <xs:field xpath="@name"/>
       </xs:unique>
   
   </xs:element>
   

Please note that nameUniqueConstraint is just an arbitrary name to identify the constraint you want.

来源：https://stackoverflow.com/questions/5687500/xml-schema-how-to-set-enumerations-to-one-appearance-per-document