Table of contents

Appendices

3.3 Global vs. Local Declarations

Another authoring style, applicable when all element names are unique within a namespace, is to create schemas in which all elements are global. This is similar in effect to the use of <!ELEMENT> in a DTD. In the example below, we have modified the original po1.xsd such that all the elements are declared globally. Notice that we have omitted the elementFormDefault and attributeFormDefault attributes in this example to emphasize that their values are irrelevant when there are only global element and attribute declarations.

NOTE: 

Modified version of po1.xsd using only global element declarations

<schema xmlns="http://www.w3.org/2001/XMLSchema"
        xmlns:po="http://www.example.com/PO1"
        targetNamespace="http://www.example.com/PO1">

  <element name="purchaseOrder" type="po:PurchaseOrderType"/>

  <element name="shipTo"  type="po:USAddress"/>
  <element name="billTo"  type="po:USAddress"/>
  <element name="comment" type="string"/>

  <element name="name" type="string"/>
  <element name="street" type="string"/>

  <complexType name="PurchaseOrderType">
    <sequence>
      <element ref="po:shipTo"/>
      <element ref="po:billTo"/>
      <element ref="po:comment" minOccurs="0"/>
      <!-- etc. -->
    </sequence>
  </complexType>

  <complexType name="USAddress">
    <sequence>
      <element ref="po:name"/>
      <element ref="po:street"/>
      <!-- etc. -->
    </sequence>
  </complexType>

  <!-- etc. -->

</schema>

This "global" version of po1.xsd will validate the instance document po2.xml which, as we described previously, is also schema valid against the "qualified" version of po1.xsd. In other words, both schema approaches can validate the same, namespace defaulted, document. Thus, in one respect the two schema approaches are similar, although in another important respect the two schema approaches are very different. Specifically, when all elements are declared globally, it is not possible to take advantage of local names. For example, you can only declare one global element called "title". However, you can locally declare one element called "title" that has a string type, and is a subelement of "book". Within the same schema (target namespace) you can declare a second element also called "title" that is an enumeration of the values "Mr Mrs Ms".