Virtuoso supports creating XML views by using annotated XSD schemas referred to as mapping schemas. A file containing a mapping schema may be loaded by calling the xml_load_mapping_schema_decl() function. A name (without extension .xsd) of the file containing a mapping schema is considered to be the name of the xml view, defined by the given mapping schema.

A loaded mapping schema may be queried in the same way as one would query XML views defined using the CREATE XML VIEW statement with XPATH:

XPATH [__view "xml_view_name"]/xpath_query

   select  xml_load_mapping_schema_decl (
      'http://localhost.localdomain/xmlrepository/',
      'Catmp.xsd',
      'UTF-8',
      'x-any' ) ) );

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns:sql="urn:schemas-microsoft-com:mapping-schema">
<xsd:annotation>
  <xsd:appinfo>
    <sql:relationship name="CategoryProduct"
          parent="Demo.demo.Categories"
          parent-key="CategoryID"
          child="Demo.demo.Products"
          child-key="CategoryID" />
  </xsd:appinfo>
</xsd:annotation>

  <xsd:element name="category" sql:relation="Demo.demo.Categories" type="CategoryType" />
   <xsd:complexType name="CategoryType" >
     <xsd:sequence>
        <xsd:element name="product"
                     sql:relation="Demo.demo.Products"
                     sql:relationship="CategoryProduct" >
           <xsd:complexType>
              <xsd:attribute name="ProductName" type="xsd:string" />
           </xsd:complexType>
        </xsd:element>
     </xsd:sequence>
        <xsd:attribute name="CategoryID"  type="xsd:integer" />
        <xsd:attribute name="description"  sql:field="Description"  type="xsd:string" />
    </xsd:complexType>
</xsd:schema>
                        

XPATH [__view 'Catmp'] /category[@* = 1];
                        

<category CategoryID="1" description="Soft drinks, coffees, teas, beers, and ales" >
  <product ProductName="Chai" ></product>
  <product ProductName="Chang" ></product>
  <product ProductName="Guarana Fantastica" ></product>
  <product ProductName="Sasquatch Ale" ></product>
  <product ProductName="Steeleye Stout" ></product>
  <product ProductName="Cote de Blaye" ></product>
  <product ProductName="Chartreuse verte" ></product>
  <product ProductName="Ipoh Coffee" ></product>
  <product ProductName="Laughing Lumberjack Lager" ></product>
  <product ProductName="Outback Lager" ></product>
  <product ProductName="Rhonbrau Klosterbier" ></product>
  <product ProductName="Lakkalikoori" ></product>
</category>

Mapping schemas provide more flexibility than XML views defined by the CREATE XML VIEW statement. In the following mapping schema a constant element, "CustomerOrders", an element that does not map to any database table or column but may appear in the resulting XML as a container element of other subelements, is specified by the sql:is-constant annotation.

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            xmlns:sql="urn:schemas-microsoft-com:mapping-schema">
<xsd:annotation>
  <xsd:appinfo>
    <sql:relationship name="CustOrders"
        parent="Demo.demo.Customers"
        parent-key="CustomerID"
        child="Demo.demo.Orders"
        child-key="CustomerID" />
  </xsd:appinfo>
</xsd:annotation>

  <xsd:element name="Customer" sql:relation="Demo.demo.Customers" >
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element name="CustomerOrders" sql:is-constant="1" >
          <xsd:complexType>
            <xsd:sequence>
              <xsd:element name="Order" sql:relation="Demo.demo.Orders"
                           sql:relationship="CustOrders"
                           maxOccurs="unbounded" >
                <xsd:complexType>
                   <xsd:attribute name="OrderID" type="xsd:integer" />
                   <xsd:attribute name="OrderDate" type="xsd:date" />
                   <xsd:attribute name="CustomerID" type="xsd:string" />
                </xsd:complexType>
              </xsd:element>
            </xsd:sequence>
          </xsd:complexType>
        </xsd:element>
      </xsd:sequence>
          <xsd:attribute name="CustomerID" type="xsd:string" />
    </xsd:complexType>
  </xsd:element>
</xsd:schema>

After loading the "Cust_constant.xsd" file containing the given mapping schema, the xpath query:

XPATH [__view 'Cust_constant'] /category[@* = 1];
                        

will return the following result:

<Customer CustomerID="ALFKI" >
  <CustomerOrders >
    <Order CustomerID="ALFKI" OrderDate="1995-09-25 00:00:00.000000" OrderID="10643" ></Order>
    <Order CustomerID="ALFKI" OrderDate="1995-11-03 00:00:00.000000" OrderID="10692" ></Order>
    <Order CustomerID="ALFKI" OrderDate="1995-11-13 00:00:00.000000" OrderID="10702" ></Order>
    <Order CustomerID="ALFKI" OrderDate="1996-02-15 00:00:00.000000" OrderID="10835" ></Order>
    <Order CustomerID="ALFKI" OrderDate="1996-04-15 00:00:00.000000" OrderID="10952" ></Order>
    <Order CustomerID="ALFKI" OrderDate="1996-05-09 00:00:00.000000" OrderID="11011" ></Order>
  </CustomerOrders>
</Customer>
    . . .

Virtuoso does not support all mapping schema annotations at this time. The following are currently unsupported:

Also, there are some restrictions to the structure of mapping schemas:

	Note:
	The XML views, defined by mapping schemas may not be queried using XQUERY. They can however be referenced with the xmlview XPATH functions in path expressions inside an XQuery query.

	See Also:
	SQLXML 3.0 documentation: Creating XML Views by Using Annotated XSD Schemas.