求助：TreeView如何绑定数据库

//.aspx的代码如下，也是我从网上看到得

//数据库：一级目录：FirstCategories:FCatID,FCatName;

//二级目录：SecondCategories:SCatID,SCatName,FCatID

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Configuration;
using System.Data.SqlClient;

namespace WebApplication1
{
public partial class WebForm1 : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
DataTable dt = GetFromDB();
LoadCategories(null, "0", dt);
}
}
//菜单得加载
public void LoadCategories(TreeNode tn, string pid, DataTable dt)
{
DataRow[] rows = dt.Select("FCatID=" + pid);
foreach (DataRow row in rows)
{
TreeNode node = new TreeNode();
node.Text = row["SCatName"].ToString();
node.Value = row["SCatID"].ToString();
if (tn == null)
{
this.TreeView1.Nodes.Add(node);
}
else
{
tn.ChildNodes.Add(node);
}
LoadCategories(node, node.Value, dt);
}
}

//从数据库中获得信息
public DataTable GetFromDB()
{
DataTable dt = new DataTable();
string connStr = ConfigurationManager.ConnectionStrings["connection"].ConnectionString;
using (SqlConnection conn = new SqlConnection(connStr))
{
conn.Open();
string sql = "select FirstCategories.FCatID,FCatName,SCatID,SCatName from FirstCategories inner join  SecondCategories on FirstCategories.FCatID=SecondCategories.FCatID ";
SqlCommand comm = new SqlCommand(sql, conn);
SqlDataAdapter da = new SqlDataAdapter(comm);
da.Fill(dt);
}
return dt;
}
}