XML schema, why xs:group can't be child of xs:all?

问题

According to this page (and my practice), xs:group element cannot be child of xs:all. So something like

<xs:group name="g">
    <xs:element name="first" type="xs:string"/>
    <xs:element name="last" type="xs:string"/>
</xs:group>
<xs:all>
    <xs:group ref="g" minOccurs="0" maxOccurs="1"/>
    <xs:element name="id" type="xs:string"/>
</xs:all>

is not valid because group cannot be inside xs:all. But I want to define a schema, in which two elements (first and last in above example) both exist or neither of them exists, so I make them into a group. Then I want to make the group part of xs:all because the group can appear with other elements (for example, id element above) in any order. In other words, I want to have several elements are optional as a whole group. Without xs:group being able to be child of xs:all, how can I achieve this?

回答1:

XML Schema 1.0 only permits xs:element (and xs:annotation) under xs:all.

<all
  id = ID
  maxOccurs = 1 : 1
  minOccurs = (0 | 1) : 1
  {any attributes with non-schema namespace . . .}>
  Content: (annotation?, element*)
</all>

It does not allow xs:group, xs:sequence, xs:choice etc.

XML Schema 1.1 permits xs:element, xs:any, or xs:group under xs:all:

<all
  id = ID
  maxOccurs = (0 | 1) : 1
  minOccurs = (0 | 1) : 1
  {any attributes with non-schema namespace . . .}>
  Content: (annotation?, (element | any | group)*)
</all>

Note: Allowing unordered elements may sound ideal but rarely is truly needed. Usually xs:sequence suffices in practice.

If you are willing to forgo the unordered requirement, you could (even in XSD 1.0) require first and last to "both exist or neither of them exist" as follows:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="a">
    <xs:complexType>
      <xs:sequence>
        <xs:sequence minOccurs="0">
          <xs:element name="first" type="xs:string"/>
          <xs:element name="last" type="xs:string"/>
        </xs:sequence>
        <xs:element name="id" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

来源：https://stackoverflow.com/questions/33698600/xml-schema-why-xsgroup-cant-be-child-of-xsall

标签

xml

xslt

xsd