In the first article in this series, we took a look at the element vs. attribute question for data points and concluded that, for data documents, it makes more sense to model the information as attributes when possible. In this, the second part of our four-part series, we take a look at the issues involved when mapping relational data points (columns) to their XML Schema equivalents.

In this and following articles, we assume that the xsd namespace is mapped to the namespace for XML Schemas v. 1.0: http://www.w3.org/2001/XMLSchema.

Representing data points in XML Schemas
When creating your XML Schema from relational data, each column that contains information you want to model in your schema must have a corresponding data point declaration. As we saw in the first article, there are two ways of doing this: we can model the data point as an element or as an attribute. Let's take a look at how this is done.

Modeling data points as elements
A data point is modeled in an XML Schema using the <xsd:element> element. This element is used to model both elements that contain simple values (data points) and elements that contain further structure—another reason to consider using attributes instead of elements for this purpose. There are a couple of ways you can approach the modeling of the data point: you can either define a global element and then reference it inside your container element using the ref attribute, or you can model the element in place using the name and type attributes. Let's take a look at each mechanism.

Using the name and type attributes
When you model your data points this way, you declare your data point element with the name attribute set to the name of the element containing the datapoint, and the type attribute set to the type of the value contained in the element. This element is then declared in the context of a complexType, either a named complexType or an anonymous complexType declared as part of an element declaration. (Don't worry too much about this distinction right now—we'll learn more about this in the next article). For example, let's say we have a table called Customer we want to model in an XML Schema. The SQL for the table looks something like this:

    CREATE TABLE Customer (


     customerID int,


     customerName varchar(200)


     )

To model the customerName point in our XML Schema using name and type, we would use the following sample element declaration:

<xsd:schema

    xmlns:xsd="http://www.w3.org/2001/XMLSchema">


     <xsd:element name="customer">


     <xsd:complexType>


     <xsd:sequence>


     <xsd:element name="customerName" type="xsd:string"

    />


     </xsd:sequence>


     </xsd:complexType>


     </xsd:element>


    </xsd:schema>

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