Back to Our Library (XML Schema)

4.8. Back to Our Library

If we look back with a critical eye at our library, we see we used the following simple datatypes:

<xs:element name="name" type="xs:string"/>
      
<xs:element name="qualification" type="xs:string"/>
      
<xs:element name="born" type="xs:date"/>
      
<xs:element name="dead" type="xs:date"/>
      
<xs:element name="isbn" type="xs:string"/>
      
<xs:attribute name="id" type="xs:ID"/>
      
<xs:attribute name="available" type="xs:boolean"/>
      
<xs:attribute name="lang" type="xs:language"/>

We are lucky that the elements born and dead are ISO 8601 dates. The ISBN number is composed of numeric digits and a final character which can be either a digit or the letter "x"-and is therefore represented as a string. We also did a good job with the datatypes for the id, available and lang attributes, but the choice of xs:string for the elements name and qualification is more controversial. They appear in the instance document as:

<name>
  Charles M Schulz
</name>
                      .../...

<qualification>
  bold, brash and tomboyish
</qualification>

This formatting suggests that whitespaces are probably not significant and should be collapsed. This can be done by choosing the datatype xs:token instead of xs:string; the same applies to the title element, which is a simple content derived from xs:string that would be better derived from xs:token. This change will not have any impact on the validation with our schema, but the document is more precisely described and future derivations would be more easily built on xs:token than on xs:string. The other datatype that could have been chosen better is isbn, which can be represented as xs:NMTOKEN. The new schema would then be:

<?xml version="1.0"?> 
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="name" type="xs:token"/>
  <xs:element name="qualification" type="xs:token"/>
  <xs:element name="born" type="xs:date"/>
  <xs:element name="dead" type="xs:date"/>
  <xs:element name="isbn" type="xs:NMTOKEN"/>
  <xs:attribute name="id" type="xs:ID"/>
  <xs:attribute name="available" type="xs:boolean"/>
  <xs:attribute name="lang" type="xs:language"/>
  <xs:element name="title">
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base="xs:token">
          <xs:attribute ref="lang"/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>
  </xs:element>
  <xs:element name="library">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="book" maxOccurs="unbounded"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="author">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="name"/>
        <xs:element ref="born"/>
        <xs:element ref="dead" minOccurs="0"/>
      </xs:sequence>
      <xs:attribute ref="id"/>
    </xs:complexType>
  </xs:element>
  <xs:element name="book">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="isbn"/>
        <xs:element ref="title"/>
        <xs:element ref="author" minOccurs="0" maxOccurs="unbounded"/> 
        <xs:element ref="character" minOccurs="0"
          maxOccurs="unbounded"/>
      </xs:sequence>
      <xs:attribute ref="id"/>
      <xs:attribute ref="available"/>
    </xs:complexType>
  </xs:element>
  <xs:element name="character">
    <xs:complexType>
      <xs:sequence>
        <xs:element ref="name"/>
        <xs:element ref="born"/>
        <xs:element ref="qualification"/>
      </xs:sequence>
      <xs:attribute ref="id"/>
    </xs:complexType>
  </xs:element>
</xs:schema>