https://www.xmls.cn/home.aspx XML Schema union 元素

<union
id=ID
memberTypes="list of QNames"
any attributes
>

(annotation?,(simpleType*))

</union>

<xs:element name="jeans_size">
  <xs:simpleType>
    <xs:union memberTypes="sizebyno sizebystring" />
  </xs:simpleType>
</xs:element>

<xs:simpleType name="sizebyno">
  <xs:restriction base="xs:positiveInteger">
    <xs:maxInclusive value="42"/>
  </xs:restriction>
</xs:simpleType>

<xs:simpleType name="sizebystring">
  <xs:restriction base="xs:string">
    <xs:enumeration value="small"/>
    <xs:enumeration value="medium"/>
    <xs:enumeration value="large"/>
  </xs:restriction>
</xs:simpleType>