Table of contents

Appendices

2.8 Attribute Groups

Suppose we want to provide more information about each item in a purchase order, for example, each item's weight and preferred shipping method. We can accomplish this by adding weightKg and shipBy attribute declarations to the item element's (anonymous) type definition:

NOTE: 

Adding Attributes to the Inline Type Definition

<xsd:element name="Itemitem" minOccurs="0" maxOccurs="unbounded">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element   name="productName" type="xsd:string"/>
      <xsd:element   name="quantity">
        <xsd:simpleType>
          <xsd:restriction base="xsd:positiveInteger">
            <xsd:maxExclusive value="100"/>
          </xsd:restriction>
        </xsd:simpleType>
      </xsd:element>
      <xsd:element name="USPrice"  type="xsd:decimal"/>
      <xsd:element ref="comment"   minOccurs="0"/>
      <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
    </xsd:sequence>
    <xsd:attribute name="partNum"  type="SKU" use="required"/>
    <!-- add weightKg and shipBy attributes -->
    <xsd:attribute name="weightKg" type="xsd:decimal"/>
    <xsd:attribute name="shipBy">
      <xsd:simpleType>
        <xsd:restriction base="xsd:string">
          <xsd:enumeration value="air"/>
          <xsd:enumeration value="land"/>
          <xsd:enumeration value="any"/>
        </xsd:restriction>
      </xsd:simpleType>
    </xsd:attribute>
  </xsd:complexType>
</xsd:element>

ref19Alternatively, we can create a named attribute group containing all the desired attributes of an item element, and reference this group by name in the item element declaration:

NOTE: 

Adding Attributes Using an Attribute Group

<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
  <xsd:complexType>
    <xsd:sequence>
      <xsd:element name="productName" type="xsd:string"/>
      <xsd:element name="quantity">
        <xsd:simpleType>
          <xsd:restriction base="xsd:positiveInteger">
            <xsd:maxExclusive value="100"/>
          </xsd:restriction>
        </xsd:simpleType>
      </xsd:element>
      <xsd:element name="USPrice"  type="xsd:decimal"/>
      <xsd:element ref="comment"   minOccurs="0"/>
      <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
    </xsd:sequence>

    <!-- attributeGroup replaces individual declarations -->
    <xsd:attributeGroup ref="ItemDelivery"/>
  </xsd:complexType>
</xsd:element>

<xsd:attributeGroup id="ItemDelivery">
  <xsd:attribute name="partNum"  type="SKU" use="required"/>
  <xsd:attribute name="weightKg" type="xsd:decimal"/>
  <xsd:attribute name="shipBy">
    <xsd:simpleType>
      <xsd:restriction base="xsd:string">
        <xsd:enumeration value="air"/>
        <xsd:enumeration value="land"/>
        <xsd:enumeration value="any"/>
      </xsd:restriction>
    </xsd:simpleType>
  </xsd:attribute>
</xsd:attributeGroup>

Using an attribute group in this way can improve the readability of schemas, and facilitates updating schemas because an attribute group can be defined and edited in one place and referenced in multiple definitions and declarations. These characteristics of attribute groups make them similar to parameter entities in XML 1.0. Note that an attribute group may contain other attribute groups. Note also that both attribute declarations and attribute group references must appear at the end of complex type definitions.