public void addCoupon(Coupon coupon)
        {
            using (var connection = new SQLiteConnection(connString))
            {
                connection.Open();
                string addCouponQuerry = String.Format(@"INSERT INTO COUPONS
                                                        (COUPON_NUMBER, COUPON_TYPE, COUPON_DISCOUNT) VALUES
                                                        ('{0}', '{1}', '{2}')", coupon.NumberOfCoupon,
                                                         coupon.TypeOfCoupon, coupon.Discount);

                SQLiteCommand addCouponCommand = new SQLiteCommand(addCouponQuerry, connection);
                addCouponCommand.ExecuteNonQuery();
            }
        }
        private void Button_Coupon_Add_Click(object sender, RoutedEventArgs e)
        {
            if (couponNumber.Text.Equals("") || Discount.Text.Equals(""))
            {
                MessageBox.Show("Not all fields are filled!");
            }
            else
            {
                ComboBoxItem typeItem = (ComboBoxItem)comboBox.SelectedItem;

                Coupon coupon = new Coupon(Convert.ToInt32(couponNumber.Text), Convert.ToDouble(Discount.Text), typeItem.Content.ToString());
                WorkWithDatabase work = new WorkWithDatabase();
                work.addCoupon(coupon);
                couponNumber.Text = "";
                Discount.Text = "";

            }
        }