returning XML Document through C# Web Service

I don't know where you stored your table. But one thing you should do that, create webservice method that return a Dataset. After you create the webservice, then consume it you application. The point that you need to have an XML output. To get this XML call the DataSet.WriteXML(string fileName) method.

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
public class WebService : System.Web.Services.WebService
{
    public WebService()
    {
        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }
    [WebMethod]
    public DataSet GetData()
    {
         DataSet dataSet = new DataSet();
        using (SqlConnection connection = new SqlConnection("connectionString"))
        {
            connection.Open();
            using (SqlCommand command = new SqlCommand("SQL Statement",connection))
            {
                SqlDataAdapter adapter = new SqlDataAdapter(command);
                adapter.Fill(dataSet);
            }
        }       
        return dataSet;
    }
}

Web service consumer application say Asp.Net

protected void Page_Load(object sender, EventArgs e)
{
   WebService ws = new WebService();
   DataSet dataSet= ws.GetData();
   dataSet.WriteXml("c:\\exportToXML.xml");
}

I hope this will help you.