Here i will explain the binding of a treeview from the Database.This can be a tricky task since we may have to build a parent-child relationship here.So first thing you would need is a table in the database where we actually have the parent-child relationship.
An Example
Table Name: Category
CategoryId  INT
CategoryName  Varchar(50)
ParentCategoryId INT
NodeLevel  INT

Suppose we have Data in the Table named Category as follows

Now to bind this to the tree,we will use a recursive function.Also for better performance,we will bind the database results into a global DataTable first and then apply filtering.

Here is the Code to bind to the DataTable.

 private void bindDataTable()
    {
       
        string strConn = ConfigurationManager.ConnectionStrings["yourConnectionName"].ConnectionString;
        SqlConnection conn = new SqlConnection(strConn);
        SqlCommand cmd = new SqlCommand("Select * from Category", conn);
        cmd.CommandType = CommandType.Text;
       
        SqlDataAdapter sqlDa = new SqlDataAdapter();
        sqlDa.SelectCommand = cmd;
        sqlDa.Fill(dt);
        conn.Close();

    }

Now we will add a recursive method that will make use of this DataTable to build the TreeView.

private void buildNodes(TreeNode n, int? categoryId)
    {
        DataRow[] drCategories;
        if (categoryId == null)
            drCategories = dt.Select("NodeLevel=0"); // This will get the Root Node
        else
            drCategories = dt.Select("ParentIdFK='" + categoryId + "'");

        foreach (DataRow dr in drCategories)
        {
            TreeNode t = new TreeNode();
            t.Text = dr["CategoryName"].ToString();
            t.Value = dr["CategoryId"].ToString();    
           
            buildNodes(t, Convert.ToInt32(dr["CategoryId"].ToString()));// Calling the function resursively
            if (n == null)// If it is Root Node
                TreeView1.Nodes.Add(t);
            else
                n.ChildNodes.Add(t);

        }

Above we are using int? so that int type can accept null values.

So now all we have to do is call these methods.

  protected void Page_Load(object sender, EventArgs e)
     {
         if (!IsPostBack)
         {
              bindDataTable();
              buildNodes(null, null);//CategoryId=null is passed which is the ParentId of the Root Node
         }
     }

Now you can add some formatting and Css as you may please to get a better looking Treeview.Happy Coding Folks!!!

Related posts:

1. How to Bind Images from Database in a GridView
2. Edit,Update,Cancel In a GridView
3. Dropdownlist inside a Gridview
4. Creating your first WebService using .NET
5. Multiview and View controls in ASP.NET(3.5)