Back to Our Library (XML Schema)

5.5. Back to Our Library

Let's see how we can improve our schema by adding constraints on our datatypes with what we have learned in this chapter:

<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"/>

First, we may want to limit the size of our strings--for instance, if they must be stored into fixed-length columns in an RDBMS. Here, we will consider that the name needs to fit in a string of 32 characters, and the title and qualification need to fit in strings of 255 characters. We create two simple datatypes for this:

<xs:simpleType name="string255">
  <xs:restriction base="xs:token">
    <xs:maxLength value="255"/>
  </xs:restriction>
</xs:simpleType>
      
<xs:simpleType name="string32">
  <xs:restriction base="xs:token">
    <xs:maxLength value="32"/>
  </xs:restriction>
</xs:simpleType>

Then, we may want to add some constraints on the ISBN number. The best we can do without using the patterns (we will see how to do this in the next chapter) is to limit the number of characters to 10 using xs:length. This facet is a number of characters and acts on the value space. This, therefore, does not eliminate instances such as ABCDEFGHIJ, but this is probably the best we can do for the moment:

<xs:simpleType name="isbn">
  <xs:restriction base="xs:NMTOKEN">
    <xs:length value="10"/>
  </xs:restriction>
</xs:simpleType>

We may finally want to limit the languages in which the title may be written. If our library only has titles in English and Spanish, we can add the following restriction:

<xs:simpleType name="supportedLanguages">
  <xs:restriction base="xs:language">
    <xs:enumeration value="en"/>
    <xs:enumeration value="es"/>
  </xs:restriction>
</xs:simpleType>

Our new schema is then:

<?xml version="1.0"?> 
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:simpleType name="string255">
    <xs:restriction base="xs:token">
      <xs:maxLength value="255"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="string32">
    <xs:restriction base="xs:token">
      <xs:maxLength value="32"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="isbn">
    <xs:restriction base="xs:NMTOKEN">
      <xs:length value="10"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:simpleType name="supportedLanguages">
    <xs:restriction base="xs:language">
      <xs:enumeration value="en"/>
      <xs:enumeration value="es"/>
    </xs:restriction>
  </xs:simpleType>
  <xs:element name="name" type="string32"/>
  <xs:element name="qualification" type="string255"/>
  <xs:element name="born" type="xs:date"/>
  <xs:element name="dead" type="xs:date"/>
  <xs:element name="isbn" type="isbn"/>
  <xs:attribute name="id" type="xs:ID"/>
  <xs:attribute name="available" type="xs:boolean"/>
  <xs:attribute name="lang" type="supportedLanguages"/>
  <xs:element name="title">
    <xs:complexType>
      <xs:simpleContent>
        <xs:extension base="string255">
          <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>