DBHelper类非断开式连接数据库

//一、两种身份验证：windows身份验证、SqlServer身份验证
        string st1 = "server=.;database=orderdinner;Integrated Security = true";
        string st2 = "server=.;database=hw;uid=sa;pwd=123";

        //二、创建连接对象connection，两种连接方式:1.通过构造函数直接传身份验证字符串2.通过连接对象的属性的赋值来连接
        // SqlConnection connection1 = new SqlConnection(st2);//1
        SqlConnection connection2 = new SqlConnection();//2
        connection2.ConnectionString = st2;

            //三、打开数据库 连接对象的属性有：1.ConnectionString属性用于存储身份验证字符串2.State属性用于判断连接对象是否连接
            //连接对象的方法有：1.Open()打开方法；2.Close()关闭方法等
            connection2.Open();  

            if (connection2.State==System.Data.ConnectionState.Open)
            {
                Console.WriteLine("数据库已经连接");

                //创建command对象  向SqlCommand类申请一个对象 
                //在华为数据库中的Rank表中插入一个部门  部门编号rankID  部门名称rankName
                SqlCommand command1 = new SqlCommand("select * from rank", connection2);
                SqlCommand command2 = new SqlCommand();
                //查询
                command2.CommandText = "select * from employee inner join rank on rank.rankID=employee.rankID";
                command2.Connection = connection2;
                //command对象查询返回的是查询结果集
                SqlDataReader reader = command2.ExecuteReader();
                                
                while (reader.Read()) {
                    Console.Write(reader[0].ToString());
                    Console.Write(reader[1].ToString());
                    Console.Write(reader[2].ToString());
                    Console.Write(reader[3].ToString());
                    Console.Write(reader[4].ToString());
                    Console.Write(reader[5].ToString());
                    Console.Write(reader[6].ToString());
                    Console.Write(reader[7].ToString());
                    Console.Write(reader[8].ToString());
                    Console.WriteLine(reader["rankName"].ToString());

                }
                    reader.Close();
                     readNum.Close();
                //更新
                //Com.CommandText = "update rank set rankName='实习生' where rankId=1";
                //插入
                //Com.CommandText = "insert rank values('总管')";
                //删除
                //Com.CommandText = "delete from rank where rankId=1";
                int count=Com.ExecuteNonQuery();
                if (count > 0)
                {
                    Console.WriteLine("执行成功");
                }
                //执行带聚合函数的查询
                Com.CommandText = "select count(*) from employee";
                Object obj=Com.ExecuteScalar();
                
                    Console.WriteLine("count结果："+obj.ToString());
                
             
            }
               //四、关闭连接
                    connection2.Close();