数据库开发019 DataReader对象

    DataReader对象提供单向只读数据，只能依次读取数据，DataSet中的数据可以任意读取和修改。DataReader对象有一个很重要的方法Read，它是个布尔值，作用是读取下一条数据，当布尔值为真时执行，为假时退出。
    DataReader从数据库当中检索只读的数据流，存储在客户端的网络缓冲区当中。在内存当中只存储一行，具有开销小、速度快的特点。由于数据不在内存中缓存，所以在检索大量数据时，DataReader是一种较好的选择。DataReader只能通过Command对象ExecuteReader()方法来创建，不能实例化。
    DataReader具有独占性，如果在已经打开DataReader的情况下，将不能对Connection进行任何操作，必须在用完时调用Close()方法关闭。通过Command对象返回多个结果集，并且通过DataReader对象NextResult()方法来使用。
    DataReader对象通过Command对象的ExecuteReader()方法创建，读取DataReader对象的数据有两种方法：通过和ataGridView等数据控件绑定，直接输出；利用循环将数据取出.

Imports System.Data.SqlClient
Public Class Form1Dim conn As SqlConnectionDim da As SqlDataAdapterDim ds As DataSetPrivate Function GetConnection() As SqlConnectionReturn New SqlConnection(My.Settings.SalesConnectionString)End FunctionPrivate Sub DisplayData(ByVal str As String)DataGridView1.DataSource = Nothingconn = GetConnection()conn.Open()Dim comm As New SqlCommand(str, conn)Dim dr As SqlDataReader = comm.ExecuteReaderDim dt As New DataTabledt.Load(dr)conn.Close()DataGridView1.DataSource = dtEnd SubPrivate Sub DataReaderData()conn = GetConnection()conn.Open()Dim comm As New SqlCommand(TextBox1.Text, conn)Dim dr As SqlDataReaderdr = comm.ExecuteReaderDim strOutput As String = ""TryWhile dr.ReadstrOutput += dr("学号").ToString.PadLeft(10) + Space(5)strOutput += dr("姓名").ToString.PadLeft(10) + Space(5)strOutput += dr("语文").ToString.PadLeft(10) + Space(5)strOutput += dr("数学").ToString.PadLeft(10) + Space(5)strOutput += dr("英语").ToString.PadLeft(10) + Space(5)strOutput += vbCrLfEnd WhileCatch ex As ExceptionMsgBox("出现异常")Finallydr.Close()conn.Close()End TryLabel2.Text = strOutputEnd SubPrivate Sub UpdateData()conn = GetConnection()conn.Open()Dim comm As New SqlCommandDim trans As SqlTransactiontrans = conn.BeginTransactioncomm.Connection = conncomm.Transaction = transTrycomm.CommandText = "Update grade set 数学=95 Where 姓名 like '%周%'"comm.ExecuteNonQuery()comm.CommandText = "Update grade set 数学=65 Where 姓名 like '%张%'"comm.ExecuteNonQuery()trans.Commit()Label1.Text = "事务运行成功"Catch ex As Exceptiontrans.Rollback()Label1.Text = "有错"Finallyconn.Close()End TryEnd SubPrivate Sub InsertRecord()'conn = GetConnection()'conn.Open()'Dim strSql As String = "Insert into grade(学号,姓名,语文,数学,英语) " & _'"Values('" & txtId.Text & "','" & txtName.Text & "','" & _'txtChinese.Text & "','" & txtMaths.Text & "','" & txtEnglish.Text & "')"'Dim comm As New SqlCommand(strSql, conn)'comm.ExecuteNonQuery()'conn.Close()End SubPrivate Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.LoadDisplayData("select * from grade")TextBox1.Text = "select * from grade"End SubPrivate Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.ClickDataReaderData()End Sub
End Class