ผมต้องการเลือกข้อมลด้วย dropdownlist ตามรูปแล้วถ้าเลือก "กรุณาเลือก" ข้อมูลจะเคลียหมด เพื่อเป็นการสร้าง employee คนใหม่

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

namespace TestProgrammerCsharp
{
    public partial class EditFRM : System.Web.UI.Page
    {

        SqlConnection objConn = new SqlConnection();
        SqlCommand objCmd = new SqlCommand();
        SqlDataAdapter objAdapter;
        DataTable dt = new DataTable();
        DataSet ds = new DataSet();
        SqlDataReader objReader;
        String strConnString;

protected void Page_Load(object sender, EventArgs e)
        {
            this.txtEmployeeID.ReadOnly = true;
            strConnString = "Data Source=.\\SQL2008;Initial Catalog=XXXXXXX;Integrated Security=true";
            objConn = new SqlConnection(strConnString);
            objConn.Open();

            if (!Page.IsPostBack)
            {
                BindDataEdit();
                
                BindDataDept();
            }

        }

 private void BindDataEdit()
        {
            SqlDataAdapter objAdapter;
            DataTable dt = new DataTable();
           // string strSQL = "SELECT * FROM Employee WHERE EmployeeID ='" + Request.QueryString["EmployeeID"] + "'";
            string strSQL = "SELECT * FROM Employee e LEFT JOIN Departments d ON e.DepartmentID=d.DepartmentID WHERE e.EmployeeID=" + Request.QueryString["EmployeeID"] + "";
           

            //เรียก department ของพนักงาน
            string strSQLDept = "SELECT * FROM Departments";

            DataTable dtDept = new DataTable();

            objAdapter = new SqlDataAdapter(strSQLDept,objConn);
            objAdapter.Fill(dtDept);

            //ปิดการเรียก department ของพนักงาน


            objAdapter = new SqlDataAdapter(strSQL, objConn);
            objAdapter.Fill(dt);

            if (dt.Rows.Count > 0)
            {
                this.txtEmployeeID.Text = dt.Rows[0]["EmployeeID"].ToString();
                this.txtEmployeeName.Text = dt.Rows[0]["EmployeeName"].ToString();
                this.txtAddress.Text = dt.Rows[0]["Address"].ToString();
                this.txtTelephone.Text = dt.Rows[0]["Telephone"].ToString();

                this.lblDeptarment.Text = dt.Rows[0]["DepartmentID"].ToString();//แสดง id department ของพนักงาน

              //ผมคิดว่าบรรทัดนี้ต้อง ดึงข้อมูล Department มาใส่ใน DropdownList 
                
            }
            else
            {
                Response.Write("<b><font color='red'>ไม่พบข้อมูล</font></b>");
            }
        }
 protected void ddlDept_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (this.ddlDept.SelectedItem.Value == "0")
            {
                this.txtEmployeeID.Text = "";
                this.txtEmployeeID.ReadOnly = false;
                
                this.txtEmployeeName.Text = "";
                this.txtAddress.Text = "";
                this.txtTelephone.Text = "";
                this.btnCreate.Visible =true;
            } 

            
        }
        void BindDataDept()
        {
            ddlDept.Items.Clear();

            string selectSQLDept;
            selectSQLDept = "SELECT * FROM Departments";

            objConn = new SqlConnection(strConnString);
            objCmd = new SqlCommand(selectSQLDept, objConn);
            SqlDataReader reader;
            try
            {
                objConn.Open();
                reader = objCmd.ExecuteReader();

                // For each item, add the author name to the displayed
                // list box text, and store the unique ID in the Value property.
                while (reader.Read())
                {
                    ListItem newItem = new ListItem();
                    newItem.Text = reader["DepartmentName"].ToString();
                    newItem.Value = reader["DepartmentID"].ToString();
                    ddlDept.Items.Add(newItem);
                }
                reader.Close();
            }
            catch (Exception err)
            {
                //lblResultDept.Text = "Error reading list of DepartmentName.";
                //lblResultDept.Text += err.Message;
            }
            finally
            {
                objConn.Close();
            }
        }

        protected void btnEdit_Click(object sender, EventArgs e)
        {

            if (this.ddlDept.SelectedItem.Value=="0")
            {
                Response.Write("<b><font color='red'>กรุณาเลือกแผนกก่อนสร้างข้อมูล!!</font></b>");
            }
            else
            {
                string strSQL = "UPDATE Employee SET " +
               " EmployeeName ='" + this.txtEmployeeName.Text + "'" +
               " ,Address ='" + this.txtAddress.Text + "'" +
               " ,Telephone ='" + this.txtTelephone.Text + "'" +
               " ,DepartmentID ='" + this.ddlDept.SelectedItem.Value + "'" +
               " WHERE EmployeeID ='" + Request.QueryString["EmployeeID"] + "'";

                objCmd = new SqlCommand();
                objCmd.Connection = objConn;
                objCmd.CommandText = strSQL;
                objCmd.CommandType = CommandType.Text;


                try
                {
                    objCmd.ExecuteNonQuery();
                    Response.Write("<b><font color='red'>Update successful</font></b>");
                }
                catch (Exception ex)
                {

                    Response.Write("<b><font color='red'>Update failed  (" + ex.Message + ")</font></b>");
                }

            }
            


        }

    }
}