1.ExecuteScalar返回单个值，如执行COUNT（*），当我们查询结果只有一个值时，通常用这个方法。

 //查看连接
        private void button1_Click(object sender, EventArgs e)
        {
            string connection = "Persist Security Info=False;Integrated Security=SSPI;Initial Catalog=TestConnection;server=.\\SQLEXPRESS";
            SqlConnection con = new SqlConnection(connection);
            con.Open();
            SqlCommand com = con.CreateCommand();
            com.CommandText = "select count(*) from wachyy";
            int i = (int)com.ExecuteScalar();
            MessageBox.Show(i.ToString ());
        }

2。Data读取数据时，不能对它们进行修改，只读的

 public List<Bank> GetAllUserInfo()
        {
            List<Bank> lb = new List<Bank>();
            string conString = ConfigurationManager.ConnectionStrings["myConnection"].ToString();
            using (SqlConnection con = new SqlConnection(conString))
            {
                SqlCommand com = con.CreateCommand();
                con.Open();
                com.CommandType = CommandType.Text;
                com.CommandText = "select * from wachyy";
                SqlDataReader sdr = com.ExecuteReader();
                if (sdr.HasRows)
                {
                    while (sdr.Read())
                    {
                        Bank bank = new Bank();
                        bank.UserName = sdr["userName"].ToString();
                        bank.CurrentMoney = Convert.ToInt32(sdr["currentMoney"]);
                        lb.Add(bank);
                    }
                }
            }

            return lb;
        }