使用ADO.NET查询和操作数据

StringBuilder类： 用来定义可变字符串
            StringBuilder sb = new StringBuilder("");
            //追加字符串
            sb.Append("World");
            sb.Append("!");
            //W2orld
            sb.Insert(2, "2");
            //原字符串：Wo2rld!      截取之后：W2rld! 
            sb.Remove(1, 2);
            //ToString()
            Console.WriteLine(sb.ToString());

查询学生记录数
  //打开数据库连接
                    con.Open();
                    //使用StringBuilder追加SQL语句
                    StringBuilder sb = new StringBuilder();
                    sb.Append("select ");
                    sb.Append(" Count(*) ");
                    sb.Append(" from ");
                    sb.Append("[Student]");
                    Console.WriteLine(sb.ToString());
                    //创建一个SqlCommand对象
                    SqlCommand com = new SqlCommand(sb.ToString(),con);
                    Console.WriteLine((int)com.ExecuteScalar());

DataReader：从数据源中检索只读、只进的数据流，每次读取一行数据

     StringBuilder sb = new StringBuilder();
                    sb.AppendLine("select");
                    sb.AppendLine("[StudentNo]");
                    sb.AppendLine(",[StudentName]");
                    sb.AppendLine("from");
                    sb.AppendLine("[Student]");
                    SqlCommand com = new SqlCommand(sb.ToString(), con);
                    //从数据源中检索只读、只进的数据流
                    return com.ExecuteReader();

                    SqlDataReader reader=GetStudentInfo();
                 while (reader.Read())
                 {
                     Console.WriteLine("{0} {1}",reader["StudentNo"],reader["StudentName"]);
                 }
                 reader.Close();

                 
ExecuteNonQuery():

      StringBuilder sb = new StringBuilder();
                    sb.AppendLine("Insert into");
                    sb.AppendLine("[Grade]([GradeName])");
                    sb.AppendLine("Values('" + gradeName + "')");
                    //3.创建一个SqlCommand
                    SqlCommand com = new SqlCommand(sb.ToString(),con);
                    //4.返回执行结果
                    return com.ExecuteNonQuery();

                    Student stu = new Student();
              Console.WriteLine("请输入年级名称：");
              string gradename = Console.ReadLine();
              int count = stu.AddGrade(gradename);
              if (count > 0)
               {
                   Console.WriteLine("success！");
               }
              else
              {
                  Console.WriteLine("success mother!");
              }