How to select the data from dropdownlist and display that particular records in another dropdown list

protected void Page_Load(object sender, EventArgs e)
   {
        
        if (!IsPostBack)
        {
             FillDropDownList();
        }
    }     

private void FillDropDownList()
      {
          DataSet ds = new DataSet();
          SqlDataAdapter myda = new SqlDataAdapter("Select company  FROM Tablename", connection Object);
     myda.Fill(ds);
     drop_company.DataSource = ds;
     drop_company.DataValueField = "company";
     drop_company.DataBind();
     drop_company.Items.Insert(0, new ListItem("Select", "0"));
 }

protected void drop_company_SelectedIndexChanged(object sender, EventArgs e)
   {

       DataSet ds = new DataSet();
       SqlDataAdapter myda = new SqlDataAdapter("Select department FROM tablename where company='"+drop_company.SelectedItem.Value+"'",connection_Object);
       myda.Fill(ds);
       drop_dept.DataSource = ds;
       drop_dept.DataValueField = "department";
       drop_dept.DataBind();
       drop_dept.Items.Insert(0, new ListItem("Select", "0"));

   }