XML Structures for Existing Databases, cont.

Adding ID Attributes

The next step is to create an ID attribute for each of the structural (nondata point) elements we have defined so far in our XML database (with the exception of the root element). This is to uniquely identify elements that need to be referred to by other elements.

For the name of the attribute, we use the element name followed by ID. This might cause name collisions with other attributes that have already been added to the XML, in which case we need to change the names of these as appropriate. These should be defined as being of type ID, and must be declared as #REQUIRED. If we add these ID attributes to each element for now, we can optionally remove some from the created XML structures when we come to model all of the relationships.

When populating these structures, a unique ID will need to be created for each instance of an element that is generated. We need to ensure that these IDs are unique not only across all elements of a specific type, but across all elements in our document. One way to do this programmatically (assuming that we're using automatically incremented integers for the primary keys in our database) is to use the primary key for the row being created, prefixed by the name of the table in which it appears.

For example, for the customer in our database with the ID number 17, we might use the string Customer17 for the value of the CustomerID attribute on the Customer element. If we have nonnumeric keys in our database, or similar table names with numeric suffixes (like Customer and Customer1), this may cause name collisions - as always, be on the look out for these.

In our example, then, we have:

<!ELEMENT SalesData EMPTY>

<!ATTLIST SalesData

            Status (NewVersion | UpdatedVersion | CourtesyCopy) #REQUIRED>

<!ELEMENT Invoice EMPTY>

<!ATTLIST Invoice

             InvoiceID ID #REQUIRED

             InvoiceNumber CDATA #REQUIRED

             TrackingNumber CDATA #REQUIRED

             OrderDate CDATA #REQUIRED

             ShipDate CDATA #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 EMPTY>

<!ATTLIST MonthlyTotal

             MonthlyTotalID ID #REQUIRED

             Month CDATA #REQUIRED

             Year CDATA #REQUIRED

             VolumeShipped CDATA #REQUIRED

             PriceShipped CDATA #REQUIRED>

<!ELEMENT MonthlyCustomerTotal EMPTY>

<!ATTLIST MonthlyCustomerTotal

             MonthlyCustomerTotalID ID #REQUIRED

             VolumeShipped CDATA #REQUIRED

             PriceShipped CDATA #REQUIRED>

<!ELEMENT MonthlyPartTotal EMPTY>

<!ATTLIST MonthlyPartTotal

             MonthlyPartTotalID ID #REQUIRED

             VolumeShipped CDATA #REQUIRED

             PriceShipped CDATA #REQUIRED>

<!ELEMENT LineItem EMPTY>

<!ATTLIST LineItem

             LineItemID ID #REQUIRED

             Quantity CDATA #REQUIRED

             Price CDATA #REQUIRED>

Rule 5: Add ID Attributes to the Elements.

Add an ID attribute to each of the elements we have created in our XML structure (with the exception of the root element). Use the element name followed by ID for the name of the new attribute, watching as always for name collisions. Declare the attribute as type ID, and #REQUIRED.