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Sams Teach Yourself XML in 21 Days

Sep 16, 2005

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Using XPath with a Database

In addition to storing data by using XML, we can also treat databases as XML in .NET in other ways. For example, we can address the data in a dataset by using XPath expressions, and we'll take a look at how this works here in a new Visual Basic .NET project, ch20_04.

Start by creating a new Windows project named ch20_04. In this example, you'll use the XPath expression * to get a node list of all the child elements of the <employee> element. This expression will return all the fields of the various employees in the employee table you worked with in the previous example. To create this example, start by adding to the main form in your project a button with the caption Get Names and two text boxes to display the names, as shown in Figure 20.9. You need to make the text boxes multiline text boxes by setting their Multiline property to true in the properties window at the lower right in the Visual Basic .NET development environment; you need to stretch these boxes as shown in Figure 20.9.

Figure 20.9 Creating the ch20_04 project.

In addition, create a dataset object, DataSet11, that is connected to the employee table in the pubs database, as you did in the previous example. That is, drag an OleDbDataAdapter object to the main form in the project, use the Data Adapter Configuration Wizard to connect this data adapter to the employee table, select Data, Generate Dataset to open the Generate Dataset dialog box, and click OK to create DataSet11.

Now double-click the Get Names button to open the handler method in code that will be called when the button is clicked:

Private Sub Button1_Click(ByVal sender As System.Object,
    ByVal e As System.EventArgs) Handles Button1.Click
        .
        .
        .
End Sub

When the Get Names button is clicked, you'll read the data in the dataset—the employee table—into an XmlDataDocument object, which will allow you to address that data by using XPath:

Private Sub Button1_Click(ByVal sender As System.Object,
    ByVal e As System.EventArgs) Handles Button1.Click
    DataSet11.Clear()
    OleDbDataAdapter1.Fill(DataSet11)

    Dim xmlDoc As New System.Xml.XmlDataDocument(DataSet11)
        .
        .
        .
End Sub

Now you have a new XmlDataDocument object, xmlDoc, that holds the employee table, set up explicitly as XML. Table 20.9 lists the significant properties of XmlDataDocument objects, and Table 20.10 lists the significant methods of XmlDataDocument objects.

Table 20.9. Significant Public Properties of `XmlDataDocument` Objects

Property	Description
`Attributes`	Returns the attributes of this node.
`BaseURI`	Returns the base URI of the current node.
`ChildNodes`	Returns all the child nodes of the node.
`DataSet`	Returns a dataset that contains the data in the `XmlDataDocument` object.
`DocumentElement`	Returns the root `XmlElement` object for the document.
`DocumentType`	Returns the node that contains the `DOCTYPE` declaration.
`FirstChild`	Returns the first child of the node.
`HasChildNodes`	Returns `true` if this node has any child nodes.
`InnerText`	Returns or sets the concatenated values of the node and all its child nodes.
`InnerXml`	Returns or sets the markup representing the children of the current node.
`IsReadOnly`	Returns `true` if the current node is read-only.
`Item`	Returns the given child element.
`LastChild`	Returns the last child of the node.
`LocalName`	Returns the local name of the node.
`Name`	Returns the qualified name of the node.
`NamespaceURI`	Returns the namespace URI of this node.
`NextSibling`	Returns the node immediately following this node.
`NodeType`	Returns the type of the current node.
`OuterXml`	Returns the markup representing this node and all its child nodes.
`OwnerDocument`	Returns the `XmlDocument` object to which the current node belongs.
`ParentNode`	Returns the parent of this node (for nodes that can have parents).
`Prefix`	Returns or sets the namespace prefix of this node.
`PreserveWhitespace`	Returns or sets a value indicating whether to preserve whitespace.
`PreviousSibling`	Returns the node immediately preceding this node.
`Value`	Returns or sets the value of the node.

Table 20.10. Significant Public Methods of `XmlDataDocument` Objects

Method	Description
`AppendChild`	Adds the given node to the end of the list of child nodes of the current node.
`CreateAttribute`	Creates an `XmlAttribute` object with the given name.
`CreateCDataSection`	Creates an `XmlCDataSection` object that contains the given data.
`CreateComment`	Creates an `XmlComment` object that contains the given data.
`CreateDocumentFragment`	Creates an `XmlDocumentFragment` object.
`CreateDocumentType`	Returns a new `XmlDocumentType` object.
`CreateElement`	Creates an `XmlElement` object.
`CreateNode`	Creates an `XmlNode` object.
`CreateProcessingInstruction`	Creates an `XmlProcessingInstruction` object with the given name and data.
`CreateSignificantWhitespace`	Creates an `XmlSignificantWhitespace` node.
`CreateTextNode`	Creates an `XmlText` object with the given text.
`CreateWhitespace`	Creates an `XmlWhitespace` node.
`CreateXmlDeclaration`	Creates an `XmlDeclaration` node with the given values.
`Equals`	Determines whether two object instances are equal.
`GetElementById`	Returns the `XmlElement` object that has the given ID.
`GetElementFromRow`	Retrieves the `XmlElement` object associated with the given `DataRow` object.
`GetElementsByTagName`	Returns an `XmlNodeList` object that contains a list of all descendant elements that match the given name.
`GetRowFromElement`	Retrieves the `DataRow` object associated with the given `XmlElement` object.
`GetType`	Returns the type of the current instance.
`ImportNode`	Imports a node from another document to the current document.
`InsertAfter`	Inserts the given node immediately after the given reference node.
`InsertBefore`	Inserts the given node immediately before the given reference node.
`Load`	Loads the `XmlDataDocument` object by using the given data source.
`LoadXml`	Loads the XML document from the given string.
`PrependChild`	Adds the given node to the beginning of the list of child nodes for this node.
`RemoveAll`	Removes all the child nodes and/or attributes of the current node.
`RemoveChild`	Removes the given child node.
`ReplaceChild`	Replaces the child node `oldChild` with the `newChild` node.
`Save`	Saves the XML document to the given location.
`SelectNodes`	Selects a list of nodes that match the XPath expression.
`SelectSingleNode`	Selects the first `XmlNode` object that matches the XPath expression.
`WriteTo`	Saves the `XmlDocument` object.

A XmlDataDocument object has a DocumentElement property that returns the document object of its XML documents as an XmlElement object. This XmlElement object supports a method called SelectNodes that lets us select XML nodes by using XPath expressions. To select all the employee elements in the employee table, all you have to do is pass the SelectNodes method the XPath expression *, and you get back a NodeList object back that holds the matching nodes:

Private Sub Button1_Click(ByVal sender As System.Object,
    ByVal e As System.EventArgs) Handles Button1.Click
    DataSet11.Clear()
    OleDbDataAdapter1.Fill(DataSet11)

    Dim xmlDoc As New System.Xml.XmlDataDocument(DataSet11)

    Dim nodeList As System.Xml.XmlNodeList =

           xmlDoc.DocumentElement.SelectNodes("*")
        .
        .
        .
End Sub

Now you have a node list of the <employee> elements, extracted from your XmlDataDocument using XPath. How can you access the data in the corresponding employee records in the dataset? You can do that with the XmlDataDocument object's GetRowFromElement method—all you have to do is to pass an XML element to this method, and it'll return the dataset row containing the employee's record. When you have the employee's data record, you can extract the employee's first and last names (Fields 1 and 3 in each record) this way:

Private Sub Button1_Click(ByVal sender As System.Object,
    ByVal e As System.EventArgs) Handles Button1.Click
    DataSet11.Clear()
    OleDbDataAdapter1.Fill(DataSet11)

    Dim xmlDoc As New System.Xml.XmlDataDocument(DataSet11)

    Dim nodeList As System.Xml.XmlNodeList =
        xmlDoc.DocumentElement.SelectNodes("*")
    Dim tempRow As DataRow

       Dim tempNode As System.Xml.XmlNode


       For Each tempNode In nodeList

           tempRow = xmlDoc.GetRowFromElement(CType(tempNode,

               System.Xml.XmlElement) )


           If Not tempRow Is Nothing Then TextBox1.Text +=

               tempRow(1).ToString() + ControlChars.CrLf

           If Not tempRow Is Nothing Then TextBox2.Text +=

               tempRow(3).ToString() + ControlChars.CrLf

       Next

End Sub

When you run this example and click the Get Names button, you get all the employees records by using XPath, and then look up all the employees' first and last names and display them as shown in Figure 20.10.

Figure 20.10 Running the ch20_04 project.

Today you've used XPath to access records in a dataset, thanks to the XmlDataDocument object. Listing 20.3 shows the code that we've written for this example. Although in this example we used only the simple XPath expression * to access all the employees in the employee table, we can use full XPath expressions as well.

Example 20.3. Button-Handling Code in the `ch20_04` Project (from `Form1.vb` in the `ch20_04` Project)

Public Class Form1
    Inherits System.Windows.Forms.Form
        .
        .
        .
    Private Sub Button1_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles Button1.Click
        DataSet11.Clear()
        OleDbDataAdapter1.Fill(DataSet11)

        Dim xmlDoc As New System.Xml.XmlDataDocument(DataSet11)

        Dim nodeList As System.Xml.XmlNodeList = _
            xmlDoc.DocumentElement.SelectNodes("*")
        Dim tempRow As DataRow
        Dim tempNode As System.Xml.XmlNode

        For Each tempNode In nodeList
            tempRow = xmlDoc.GetRowFromElement(CType(tempNode, _
            System.Xml.XmlElement))

            If Not tempRow Is Nothing Then TextBox1.Text += _
                tempRow(1).ToString() + ControlChars.CrLf
            If Not tempRow Is Nothing Then TextBox2.Text += _
                tempRow(3).ToString() + ControlChars.CrLf
        Next
    End Sub
End Class

Using XPath is one way to address data in databases. However, XPath isn't powerful enough to handle databases, which is why the W3C is introducing XQuery—which is described in the next section.

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