使用ADO.NET查询和操作数据库

String和StringBuilder

语法：

//声明一个空的StringBuilder对象

StingBuilder对象名称 = new   StringBuilder();

//声明一个StringBuilder对象，值为"字符串初始值"

StringBuilder  对象名称 = new  StringBuilder("字符串的初始值"); 

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!");      

         }