title: How to load data from database to TreeView when parent and child in the same table
body: <p>I have LearningObject table in Database which contains (ID, level, subject, type).
  The Level field contains (One, Two, Three). For each level there are many subjects. 
  I need to load data from this database to TreeView: each level represents as a parent, and each subject represents as a child under the suitable level.
  The following code bellow did not work with me to implement my idea.
  I have this error
   ( An exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll but was not handled in user code
  Additional information: Unknown column 'one' in 'where clause') </p>
  
  <pre><code>protected void Page_Load(object sender, EventArgs e)
  {
      fill_Tree2();
  }
  
  public void fill_Tree2()
  {
      DataSet PrSet = PDataset("Select DISTINCT level from learningobject");
      TreeView1.Nodes.Clear();
      foreach (DataRow dr in PrSet.Tables[0].Rows)
      {
          TreeNode tnParent = new TreeNode();
          tnParent.Text = dr["level"].ToString();
          tnParent.Value = dr["level"].ToString();
          tnParent.PopulateOnDemand = true;
          tnParent.ToolTip = "Click to get Child";
          tnParent.SelectAction = TreeNodeSelectAction.SelectExpand;
          tnParent.Expand();
          tnParent.Selected = true;
          TreeView1.Nodes.Add(tnParent);
          FillChild(tnParent, tnParent.Value);
      }
  }
  
  public void FillChild(TreeNode parent, string ParentID)
  {
      DataSet ds = PDataset("Select subject,level from learningobject where level =" + ParentID.ToString());
      parent.ChildNodes.Clear();
      foreach (DataRow dr in ds.Tables[0].Rows)
      {
          TreeNode child = new TreeNode();
          child.Text = dr["subject"].ToString().Trim();
          child.Value = dr["level"].ToString().Trim();
          if (child.ChildNodes.Count == 0)
          {
              child.PopulateOnDemand = true;
          }
          child.ToolTip = "Click to get Child";
          child.SelectAction = TreeNodeSelectAction.SelectExpand;
          child.CollapseAll();
          parent.ChildNodes.Add(child);
      }
  }
  
  protected DataSet PDataset(string Select_Statement)
  {
      string connStr = @"Data Source=localhost;Database=ahsschema;User Id=webuser;Password=";
      using (MySqlConnection SqlCon = new MySqlConnection(connStr))
      {
          MySqlDataAdapter ad = new MySqlDataAdapter(Select_Statement, SqlCon);
          DataSet ds = new DataSet();
          ad.Fill(ds);
          return ds;
  
      }
  }
  </code></pre>