Elements that turn out to only be referenced by IDREF(S) need to be added as allowable element content to the root element of the DTD. Then, when creating the document, the orphaned elements are created within the root element and then pointed to, where appropriate.

Applying this rule to our example, we see that we are missing the <Customer> and <Part> elements. Adding these as allowable structural content to our root element gives us:

 
<!ELEMENT SalesData (Invoice*, Customer*, Part*, MonthlyTotal*)>
<!ATTLIST SalesData 
 Status (NewVersion | UpdatedVersion | CourtesyCopy) #REQUIRED>
<!ELEMENT Invoice (LineItem*)>
...

Rule 10: Add Missing Elements.

For any element that is only pointed to in the structure created so far, add that element as allowable element content of the root element. Set the cardinality suffix of the element being added to *.

Discard Unreferenced ID attributes

Finally, we need to discard those ID attributes that we created in Rule 5 that do not have IDREF(S) pointing to them. Since we created these attributes in the process of building the XML structures, discarding them if they are not used does not sacrifice information, and saves developers the trouble of generating unique values for the attributes.

Rule 11: Remove Unwanted ID Attributes.

Remove ID attributes that are not referenced by IDREF or IDREFS attributes elsewhere in the XML structures.

Applying Rule 11 to our example gives us our final structure. On review, the InvoiceID, LineItemID, MonthlyPartTotalID, MonthlyTotalID, and MonthlyCustomerTotalID attributes are not referenced by any IDREF or IDREFS attributes. Removing them, we arrive at our final structure, ch03_ex01.dtd:

<!ELEMENT SalesData (Invoice*, Customer*, Part*, MonthlyTotal*)>
<!ATTLIST SalesData 
 Status (NewVersion | UpdatedVersion | CourtesyCopy) #REQUIRED>
<!ELEMENT Invoice (LineItem*)>
<!ATTLIST Invoice
 InvoiceNumber CDATA #REQUIRED
 TrackingNumber CDATA #REQUIRED
 OrderDate CDATA #REQUIRED
 ShipDate CDATA #REQUIRED
 ShipMethod (USPS | FedEx | UPS) #REQUIRED
 CustomerIDREF IDREF #REQUIRED>
<!ELEMENT Customer EMPTY>
<!ATTLIST Customer
 CustomerID ID #REQUIRED
 Name CDATA #REQUIRED
 Address CDATA #REQUIRED
 City CDATA #REQUIRED
 State CDATA #REQUIRED
 PostalCode CDATA #REQUIRED>
<!ELEMENT Part EMPTY>
<!ATTLIST Part
 PartID ID #REQUIRED 
 PartNumber CDATA #REQUIRED
 Name CDATA #REQUIRED
 Color CDATA #REQUIRED
 Size CDATA #REQUIRED>
<!ELEMENT MonthlyTotal (MonthlyCustomerTotal*, MonthlyPartTotal*)>
<!ATTLIST MonthlyTotal
 Month CDATA #REQUIRED
 Year CDATA #REQUIRED
 VolumeShipped CDATA #REQUIRED
 PriceShipped CDATA #REQUIRED>
<!ELEMENT MonthlyCustomerTotal EMPTY>
<!ATTLIST MonthlyCustomerTotal
 VolumeShipped CDATA #REQUIRED
 PriceShipped CDATA #REQUIRED
 CustomerIDREF IDREF #REQUIRED>
<!ELEMENT MonthlyPartTotal EMPTY>
<!ATTLIST MonthlyPartTotal
 VolumeShipped CDATA #REQUIRED
 PriceShipped CDATA #REQUIRED
 PartIDREF IDREF #REQUIRED>
<!ELEMENT LineItem EMPTY>
<!ATTLIST LineItem
 Quantity CDATA #REQUIRED
 Price CDATA #REQUIRED
 PartIDREF IDREF #REQUIRED>

An Example XML Document

Finally, here's an example of an XML document (ch03_ex01.xml) that would be valid for this DTD:

<?xml version="1.0"?>
<!DOCTYPE SalesData SYSTEM "http://myserver/xmldb/ch03_ex01.dtd" >
<SalesData Status="NewVersion">
 <Invoice InvoiceNumber="1" 
      TrackingNumber="1" 
      OrderDate="01012000" 
      ShipDate="07012000" 
      ShipMethod="FedEx" 
      CustomerIDREF="Customer2">
   <LineItem Quantity="2" 
        Price="5" 
        PartIDREF="Part2" />
 </Invoice>
 <Customer CustomerID="Customer2" 
      Name="BobSmith" 
      Address="2AnyStreet" 
      City="Anytown" 
      State="AS" 
      PostalCode="ANYCODE" />
 <Part PartID="Part2" 
    PartNumber="13" 
    Name="Winkle" 
    Color="Red" 
    Size="10" />
 <MonthlyTotal Month="January" 
        Year="2000" 
        VolumeShipped="2" 
        PriceShipped="10">

   <MonthlyCustomerTotal VolumeShipped="5" 
              PriceShipped="25"
              CustomerIDREF="Customer2" />
   <MonthlyPartTotal VolumeShipped="8" 
            PriceShipped="40" 
            PartIDREF="Part2" />
 </MonthlyTotal>
</SalesData>