XML in a Nutshell, 3rd Edition

The complexContent Element

The preceding example actually took a shortcut with the schema language. One of the early fullName element declarations used the xs:simpleContent element to indicate that the element could only contain simple content (no nested elements). There is a corresponding content-declaration element that specifies that a complex type can only contain complex content (elements). This is the xs:complexContent element.

When the phone element was declared using an xs:complexType element with no nested element declarations, the schema processor automatically inferred that it should contain only complex content. The phone element declaration could be rewritten like so, using the xs:complexContent element:

<xs:element name="phone" minOccurs="0">
  <xs:complexType>
    <xs:complexContent>
      <xs:restriction base="xs:anyType">
        <xs:attribute name="number" type="xs:string"/>
      </xs:restriction>
    </xs:complexContent>
  </xs:complexType>
</xs:element>

The most common reason to use the xs:complexContent element is to derive a complex type from an existing type. This example derives a new type by restriction from the built-in xs:anyType type. xs:anyType is the root of all of the built-in schema types and represents an unrestricted sequence of characters and markup. Since the xs:complexType indicates that the element can only contain element content, the effect of this restriction is to prevent the element from containing either character data or markup.