C# จะให้ Combobox เลือก ข้อมูลจาก Database อย่างเดี่ยว เวลา เพิ่ม ลบ แก้ไขข้อมูล ได้ยังไงครับ

columnComboBox = new DataGridComboBoxColumn();
columnComboBox.comboBox.DataSource = dt;
columnComboBox.comboBox.DisplayMember = "Name";
columnComboBox.comboBox.ValueMember = "ID";

 private void Orderdetail_Load(object sender, EventArgs e)
        {
            dgvShow();
            buttonDefault();
        }

        private void dgvShow()
        {
            conn.ConnectionString = strConn;
            if (conn.State == ConnectionState.Open)
            {
                conn.Close();
            }
            conn.Open();

            string sql ="select [order].orderNo,product.productID,orderdetail.unit,orderdetail.price_unit,orderdetail.discount   from product,orderdetail,[order]";
            sql += "where [order].orderNo=orderdetail.orderNo and ";
            sql += "product.productID=orderdetail.productID";
          



            SqlCommand cmm = conn.CreateCommand();
            cmm.CommandText = sql;
            SqlDataReader reader = cmm.ExecuteReader();
            dgvorderdetail.Rows.Clear();
            dgvorderdetail.Columns.Clear();

            DataGridViewTextBoxColumn col1 = new DataGridViewTextBoxColumn();
            col1.HeaderText = "เลขที่ใบสั่งซื้อ";
            col1.Width = 90;


            DataGridViewTextBoxColumn col2 = new DataGridViewTextBoxColumn();
            col2.HeaderText = "รหัสสินค้า";
            col2.Width = 100;

            DataGridViewTextBoxColumn col3 = new DataGridViewTextBoxColumn();
            col3.HeaderText = "จำนวน";
            col3.Width = 100;

            DataGridViewTextBoxColumn col4 = new DataGridViewTextBoxColumn();
            col4.HeaderText = "ราคาต่อหน่วย";
            col4.Width = 125;

            DataGridViewTextBoxColumn col5 = new DataGridViewTextBoxColumn();
            col5.HeaderText = "ส่วนลด";
            col5.Width = 100;


            dgvorderdetail.Columns.AddRange(new DataGridViewColumn[] { col1, col2, col3, col4, col5 });
            int i = 0;
            while (reader.Read())
            {
                dgvorderdetail.Rows.Add();
                dgvorderdetail.Rows[i].Cells[0].Value = reader.GetValue(0).ToString();
                dgvorderdetail.Rows[i].Cells[1].Value = reader.GetValue(1).ToString();
                dgvorderdetail.Rows[i].Cells[2].Value = reader.GetValue(2).ToString();
                dgvorderdetail.Rows[i].Cells[3].Value = reader.GetValue(3).ToString();
                dgvorderdetail.Rows[i].Cells[4].Value = reader.GetValue(4).ToString();


                i++;

            }
            conn.Close();

        }

        private void buttonDefault()
        {

            btnAddSave.Text = "Add";
            btnEditUpdate.Text = "Edit";
            btnDelete.Text = "Delete";
            btnCancel.Text = "Close";
            btnAddSave.Enabled = true;
            btnEditUpdate.Enabled = true;
            btnDelete.Enabled = true;
            btnCancel.Enabled = true;
            orderNoComboBox.Items.Clear();
            productIDComboBox.Items.Clear();
            unitTextBox.Clear();
            price_unitTextBox.Clear();
            discountTextBox.Clear();


        }


        private void loadcomboBox()
        {


            orderNoComboBox.Items.Clear();
            productIDComboBox.Items.Clear();

            string strConn = "Server= ADMIN-PC\\sqlexpress;User ID=sa;Password=1234567;Database=ITProduct;Connect Timeout =120";
            SqlConnection conn = new SqlConnection();


            conn.ConnectionString = strConn;
            if (conn.State == ConnectionState.Open)
            {
                conn.Close();
            }
            conn.Open();
            string sqlorderNo = "SELECT * FROM  [order] ";

            SqlCommand cmmOrder = conn.CreateCommand();
            cmmOrder.CommandText = sqlorderNo;
            SqlDataReader readerOrder = cmmOrder.ExecuteReader();



            while (readerOrder.Read())
            {
                orderNoComboBox.Items.Add(readerOrder.GetValue(0).ToString());
            }

            conn.Close();



            conn.Open();
            string sqlproductID = "SELECT * FROM  product";

            SqlCommand cmmProduct = conn.CreateCommand();
            cmmProduct.CommandText = sqlproductID;
            SqlDataReader readerProduct = cmmProduct.ExecuteReader();

            while (readerProduct.Read())
            {
                productIDComboBox.Items.Add(readerProduct.GetValue(0).ToString());

            }

            conn.Close();

        }

        private void dgvorderdetail_CellMouseClick(object sender, DataGridViewCellMouseEventArgs e)
        {
            DataGridViewRow row = new DataGridViewRow();

            row = dgvorderdetail.Rows[e.RowIndex];


            orderNoComboBox.Text = row.Cells[0].Value.ToString();
            productIDComboBox.Text = row.Cells[1].Value.ToString();
            unitTextBox.Text = row.Cells[2].Value.ToString();
            price_unitTextBox.Text = row.Cells[3].Value.ToString();
            discountTextBox.Text = row.Cells[4].Value.ToString();
         
   
        }

        private void btnAddSave_Click(object sender, EventArgs e)
        {
            if (btnAddSave.Text == "Add")
            {
                btnAddSave.Text = "Save";
                btnCancel.Text = "Cancel";
                btnAddSave.Enabled = true;
                btnCancel.Enabled = true;
                btnDelete.Enabled = false;
                btnEditUpdate.Enabled = false;
                orderNoComboBox.Items.Clear();
                productIDComboBox.Items.Clear();
                unitTextBox.Clear();
                price_unitTextBox.Clear();
                discountTextBox.Clear();
                loadcomboBox();

            }

            else
            {
                if (unitTextBox.Text != "" && price_unitTextBox.Text != "" && discountTextBox.Text != "")
                {
                    string sql;
                    conn.ConnectionString = strConn;
                    if (conn.State == ConnectionState.Open)
                    {
                        conn.Close();
                    }

                    conn.Open();
                    sql = "select orderNo from [order] where orderNo='" +orderNoComboBox.Text + "'";
                    SqlCommand cmmorderNo = conn.CreateCommand();
                    cmmorderNo.CommandText = sql;
                    SqlDataReader rederorderNo = cmmorderNo.ExecuteReader();
                    rederorderNo.Read();
                    string orderNo = rederorderNo.GetValue(0).ToString();
                    conn.Close();
                 

                    conn.Open();
                    sql = "select productID from product where productID='" +productIDComboBox.Text + "'";
                    SqlCommand cmmproductID = conn.CreateCommand();
                    cmmproductID.CommandText = sql;
                    SqlDataReader rederproductID = cmmproductID.ExecuteReader();
                    rederproductID.Read();
                    string productID = rederproductID.GetValue(0).ToString();
                    conn.Close();
                

                    conn.Open();
                    sql = "insert into orderdetail values("+orderNo+","+productID+"," + unitTextBox.Text +","+price_unitTextBox.Text +","+discountTextBox.Text +")";
                    SqlCommand cmm = conn.CreateCommand();
                    
                    cmm.CommandText = sql;
                    cmm.ExecuteNonQuery();
                    conn.Close();
                    buttonDefault();
                    dgvShow();


                }
                else
                    MessageBox.Show("กรุณากรอกข้อมูลให้ครบ");



            }
        }