XML in a Nutshell, 3rd Edition

Controlling Element Placement

You have already seen the xs:sequence element, which dictates that the elements it contains must appear in exactly the same order in which they appear within the sequence element. In addition to xs:sequence, schemas also provide the xs:choice and xs:all elements to control the order in which elements may appear. These elements may be nested to create sophisticated element structures.

Expanding the form-letter example, a sequence adds support for various letter components to the formletter.xsd schema:

<xs:element name="letter">
  <xs:complexType mixed="true">
    <xs:sequence>
      <xs:element name="greeting"/>
      <xs:element name="body"/>
      <xs:element name="closing"/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

Now, thanks to the xs:sequence element, a letter must include a greeting element, a body element, and a closing element, in that order. But, in some cases, what is desired is that one and only one element appear from a collection of possibilities. The xs:choice element supports this. For example, if the greeting element needed to be restricted to contain only one salutation out of a permissible list, it could be declared to do so using xs:choice:

<xs:element name="greeting">
  <xs:complexType mixed="true">
    <xs:choice>
      <xs:element name="hello"/>
      <xs:element name="hi"/>
      <xs:element name="dear"/>
    </xs:choice>
  </xs:complexType>
</xs:element>

Now one of the permitted salutations must appear in the greeting element for the letter to be considered valid.

The remaining ...