XML in a Nutshell, 3rd Edition

Derived Complex Types

We have been using the xs:extension and xs:restriction elements without going too deeply into how or why they work. The schema language provides functionality for extending existing types, which is conceptually similar to that of inheritance in object-oriented programming. The extension and restriction elements allow new types to be defined either by expanding or limiting the potential values of existing types.

Deriving by extension

When deriving a new type from an existing type, the resulting type is equivalent to appending the contents of the new declaration to the contents of the base declaration. For instance, the following example declares a new type called mailingAddressType that extends the physicalAddressType type to include a Zip Code:

<xs:complexType name="mailingAddressType">
  <xs:complexContent>
    <xs:extension base="addr:physicalAddressType">
      <xs:sequence>
        <xs:element name="zipCode" type="xs:string"/>
      </xs:sequence>
    </xs:extension>
  </xs:complexContent>
</xs:complexType>

This declaration appends a required element, zipCode, to the existing physicalAddressType type. The biggest benefit of this approach is that as new declarations are added to the underlying type, the derived type will automatically inherit them.

Deriving by restriction

When a new type is a logical subset of an existing type, the xs:restriction element allows this relationship to be expressed directly. Like the xs:extension type, it allows a new type to be created based on an existing ...