sorting gridview

i want to sort my gridview according to the datakeyname id in descending order:
but the code not working
protected void Page_Load(object sender, EventArgs e) {
if (!IsPostBack) {
ViewState["sortexp"] = ""; ViewState["orderby"] = "ASC"; BindGridData();

}
}


protected void BindGridData() {
string sql = @"Data Source=SAJITH-PC\SQLEXPRESS;Initial Catalog=myfile1;Integrated Security=True"; SqlConnection con = new SqlConnection(sql); SqlCommand com = new SqlCommand();
con.Open();
SqlCommand cmd = new SqlCommand("Select top 3 id,username,dept,costcode,mobileno,pickupdate,pickuptime,fromdest,todest,notes,status from main where hodusername='" + Session["hodusername"] + "'", con); SqlDataAdapter da = new SqlDataAdapter(cmd); DataTable dt = new ...