UI（用户层），BLL（业务逻辑层），DAL（数据访问层）. 

 UI是用来显示的，比如说一些显示的样式还有界面设计。

 BLL是用来处理业务逻辑的问题。 

 DAL是和数据库打交道的。 

>数据访问层：这一层处于最底层，负责与数据库的交互，也称为DAL（Data  Access   Layer）。

>表示层：这一层直接和用户打交道，负责显示或获取数据，也称为UI层（User  Interface  Layer）。无论使用控制台还是Windows窗体显示数据，都是表示层的一种的应用。

表示层依赖于数据访问层，两层职能分开，协助工作，其依赖关系。

 他们之间的调用关系是UI调用BLL，BLL调用DAL层。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using MySchoolModel;

namespace MySchoolDAL
{
      public class Student
    {

          public bool IsLogin(string name, string pwd)
          {
              bool flag = false;
              try
              {
                  string str = "data source=.; initial Catalog=MySchool; uid=sa;pwd=mnbv";
                  SqlConnection conn = new SqlConnection(str);
                  conn.Open();
                  string sql = "select count(1) from Admin where Name=‘"+name+"‘ and Pwd=‘"+pwd+"‘";
                  SqlCommand cmd = new SqlCommand(sql,conn);
                  int num = Convert.ToInt32(cmd.ExecuteScalar());
                  if(num>0){
                      flag = true;
                  }
              }
              catch(Exception ex)
              {
                  Console.WriteLine(ex.Message);
              }
              return true;
          }
        
          public bool Add(string name,string pwd)
          {
              bool flag = false;
              try
              {
                  
                  string str = "data source=.; initial Catalog=MySchool; uid=sa;pwd=mnbv";
                  SqlConnection conn = new SqlConnection(str);
                  conn.Open();
                  string sql = "insert into Student values(‘"+name+"‘,‘"+pwd+"‘)";
                  SqlCommand cmd = new SqlCommand(sql, conn);
                  int num = Convert.ToInt32(cmd.ExecuteScalar());
                  if (num > 0)
                  {
                     
                  }
              }
              catch (Exception ex)
              {
                  Console.WriteLine(ex.Message);
              }
              return true;
          }
          public static List<Student> Const()
          {
              List<Student> list = new List<Student>();

              DataSet da = new DataSet();
              string sql = "select * from Student";
              SqlConnection conn = new SqlConnection(SqlHelper.ConStr);

              try
              {
                  conn.Open();
                  
                  SqlCommand cmd = new SqlCommand(sql, conn);
                  SqlDataReader dr = cmd.ExecuteReader();
                  while (dr.Read())
                  {
                      Student st = new Student();

                      st.name = dr["name"].ToString();
                      st.pwd = dr["pwd"].ToString();
                      
                      
                      list.Add(st);
                  }
                  dr.Close();
                  return list;
              }
              catch (Exception ex)
              {
                  throw ex;
              }
              finally
              {
                  conn.Close();
              }
              
          }

          public string name { get; set; }

          public string pwd { get; set; }
    }
}