DataSet ds = new DataSet();
ds.ReadXml(@"D:\foo.xml");
DataTable tbl =ds.Tables["cj"];
tbl.PrimaryKey = new DataColumn[] { tbl.Columns["a"] };
DataView view = tbl.DefaultView;
view.Sort = "a ASC";
DataTable sortedBy_a = view.ToTable();
//remove all the CJ tables -- they're currently unsorted
ds.Tables.Remove("cj");
//add in all the sorted CJ tables
ds.Tables.Add(sortedBy_a);
StringWriter sw = new StringWriter();
ds.WriteXml(sw);
Console.WriteLine(sw.ToString());
Console.ReadLine();