3_04_MSSQL课程_Ado.Net_.ExcuteReader()(SQLDataReader)

ExcuteNonQuery(); 返回影响的行数

ExcuteSacalar();返回第一行第一列

ExcuteReader()； Reader,指针，指向表的表头。只是指向，数据仍在数据库中。

SqlDataReader reader=cmd.ExcuteReader()；执行完之后，将指针指向数据库的表头（reader对象指向了表的表头行）默认指向表头（标题行）

reader.Read(); 指针往下走一步 ,返回的是bool类型

var str=reader["userId"]读取当前行（数据行一）列的值

 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Configuration;
 using System.Data;
 using System.Data.SqlClient;
 using System.Drawing;
 using System.IO;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
 using System.Windows.Forms;

 namespace _02ProvinceCitySelect
 {
     public partial class MainFrm : Form
     {
         public MainFrm()
         {
             InitializeComponent();
         }

         private void MainFrm_Load(object sender, EventArgs e)
         {
             //加载数据库中的所有的省的数据
             string connStr = ConfigurationManager.ConnectionStrings["LJK_SQLServerDBConnectionString"].ConnectionString;

             //创建链接对象
             using (SqlConnection conn =new SqlConnection(connStr))
             {
                 using (SqlCommand cmd =conn.CreateCommand())
                 {
                     conn.Open();//***********8
                     cmd.CommandText =@"select AreaId, AreaName, AreaPid from [dbo].[AreaFull] where AreaPId=0";
                     using (SqlDataReader reader = cmd.ExecuteReader())
                     {
                         while ( reader.Read())
                         {
                             //int AreadId = int.Parse(reader["AreaId"].ToString());
                             //把表格的数据转换成 对象数据
                             AreaInfo areaInfo = new AreaInfo();
                             areaInfo.AreaId = int.Parse(reader["AreaId"].ToString());
                             areaInfo.AreaName = reader["AreaName"].ToString();
                             areaInfo.AreaPId = int.Parse(reader["AreaPId"].ToString());
                             //把省的信息放到 ComboBox中。ComboBox显示信息是 Item对象的ToString（）
                             this.cbxProvince.Items.Add(areaInfo);
                         }
                     }//end useing  reader
                 }//end  useing cmd
             }//end  using conn

             this.cbxProvince.SelectedIndex = ;
         }

         private void cbxProvince_SelectedIndexChanged(object sender, EventArgs e)
         {
             AreaInfo provinceAreaInfo= this.cbxProvince.SelectedItem as AreaInfo;
             //判断是否 拿到的城市为空
             if (provinceAreaInfo == null)
             {
                 return;
             }
             //根据省的Id获取所有的城市信息

             //加载数据库中的所有的省的数据
             string connStr = ConfigurationManager.ConnectionStrings["LJK_SQLServerDBConnectionString"].ConnectionString;

             //创建链接对象
             using (SqlConnection conn = new SqlConnection(connStr))
             {
                 using (SqlCommand cmd = conn.CreateCommand())
                 {
                     conn.Open();//***********8
                     cmd.CommandText = @"select AreaId, AreaName, AreaPid from [dbo].[AreaFull] where AreaPId="+provinceAreaInfo.AreaId;
                     using (SqlDataReader reader = cmd.ExecuteReader())
                     {
                         this.cbxCity.Items.Clear();
                         while (reader.Read())
                         {
                             //int AreadId = int.Parse(reader["AreaId"].ToString());
                             //把表格的数据转换成 对象数据
                             AreaInfo areaInfo = new AreaInfo();
                             areaInfo.AreaId = int.Parse(reader["AreaId"].ToString());
                             areaInfo.AreaName = reader["AreaName"].ToString();
                             areaInfo.AreaPId = int.Parse(reader["AreaPId"].ToString());
                             //把省的信息放到 ComboBox中。ComboBox显示信息是 Item对象的ToString（）
                             this.cbxCity.Items.Add(areaInfo);
                         }
                     }//end useing  reader
                 }//end  useing cmd
             }//en
             this.cbxCity.SelectedIndex = ;

         }

         private void btbExport_Click(object sender, EventArgs e)
         {
             #region 选择保存文件
             string fileName = string.Empty;// 保存的文件名
             //让用户选择 要保存的文件路径
             using (SaveFileDialog sfd = new SaveFileDialog())
             {
                 if (sfd.ShowDialog() != DialogResult.OK)
                 {
                     return;                                                     //嵌套的方式，尽量少的嵌套
                 }
                 fileName = sfd.FileName;
             }
             #endregion

             // 查询数据数据，写入数据
             string connStr = ConfigurationManager.ConnectionStrings["LJK_SQLServerDBConnectionString"].ConnectionString;
             using (SqlConnection conn =new SqlConnection(connStr))
             {
                 using (SqlCommand cmd =conn.CreateCommand())
                 {
                     conn.Open();
                     cmd.CommandText = "select AreaId, AreaName, AreaPid from [dbo].[AreaFull]";
                     using (SqlDataReader reader =cmd.ExecuteReader())
                     {
                         string tempLine = string.Empty;
                         using (StreamWriter writer =new StreamWriter(fileName))     //文件流操作的学习
                         {
                             while (reader.Read())
                             {
                                 tempLine = reader["AreaId"] + "," +
                                            reader["AreaName"] + "," +
                                            reader["AreaPId"];
                                 writer.WriteLine(tempLine);//写入文本文件
                             }
                         }

                     }
                 }
             }
         }
     }
 }

reader.Read(); 指针继续往下走一行

var str=reader["userAge"]读取当前行（数据行二）列的值

在线的数据库读取，后面还有离线的数据库读取（一次性加载数据库）