计算数据库某一列的和

 private void button3_Click(object sender, EventArgs e)
        {
            string cardName = comboBox1.Text.ToString();
            double sum1 = 0;
            double sum2 = 0;
            string conStr = "server=(local);database=个人财务管理;Integrated security=true;";
            SqlConnection con = new SqlConnection(conStr);
            string sqlStr1 = "select  余额 from 银行卡信息 where 卡号尾数='" + cardName + "'";
            string sqlStr2 = "select 余额 from  银行卡信息";
            try
            {
                con.Open();
                SqlCommand com1 = new SqlCommand(sqlStr1, con);
                SqlDataReader readers1 = com1.ExecuteReader();
                while (readers1.Read())
                {
                    sum1 += double.Parse(readers1["余额"].ToString());
                }
                readers1.Close();

                SqlCommand com2 = new SqlCommand(sqlStr2, con);
                SqlDataReader readers2 = com2.ExecuteReader();
                while (readers2.Read())
                {
                    sum2 += double.Parse(readers2["余额"].ToString());
                }
                readers2.Close();
                con.Close();
            }
            catch (Exception ex5)
            {
                MessageBox.Show(ex5.Message);
            }
            textBox5.Text = "￥" + sum1.ToString();
            textBox6.Text = "￥" + sum2.ToString();
        }