private void button1_Click(object sender, EventArgs e)
        {
            string sqlcmd = "";
            for (int i = 0; i < dataGridView1.Rows.Count; i++) {
                if (dataGridView1.Rows[i].Cells[0].Value != null) {                    
                    sqlcmd += "delete from T_danhao where id = " + dataSet1.Tables[0].Rows[i][0].ToString() + ";";
                    //MessageBox.Show(sqlcmd);
                }
            }
            if (sqlcmd != "") {
                try
                {
                    Cmd = new MySqlCommand(sqlcmd, Conn);
                    Cmd.ExecuteNonQuery();
                    Fresh_table();
                }
                catch (Exception ex) {
                    MessageBox.Show(ex.Message.ToString());
                }
            }
        }