B2008 利用SQL做的一个家电下乡资料管理系统，想不添加已存在的记录，不知如何实现：
程序如下所示：（红色的的部分不会做）请高手给指点指点

Imports System.Data.SqlClient
Imports System.Data
Public Class zllrform
    Dim objconn As New SqlConnection("server=localhost;database=家电下乡资料;user id=sa;password=000159")
    Dim objdataadapter As New SqlDataAdapter()
    Dim objdataset As New DataSet()
    Dim objcommand As SqlCommand = New SqlCommand
    Dim addbut As Boolean = False
    Dim a As Integer


    Private Sub butSver_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles butSave.Click
        If ((jqxh.Text <> "") And (pmgg.Text <> "") And (bskh.Text <> "") And (cpbh.Text <> "") And (yhmc.Text <> "")) Then
            addbut = False
            objcommand.Connection = objconn
             = "select * from 家电下乡资料 " & " where 标识卡号='bskh.Text'"
            objconn.Open()
            If 这里不知判断什么 Then
                MessageBox.Show("此卡已录入，不能重复录入" & a, "添加提示", MessageBoxButtons.OK, MessageBoxIcon.Asterisk, MessageBoxDefaultButton.Button1)
                objconn.Close()
            Else
                objconn.Close()
                = "insert into 家电下乡资料" & "(机器型号,品名规格,标识卡号,产品编号,用户名称,所属公司) " & "values(@机器型号,@品名规格,@标识卡号,@产品编号,@用户名称,@所属公司)"
                objcommand.Parameters.AddWithValue("@机器型号", jqxh.Text)
                objcommand.Parameters.AddWithValue("@品名规格", pmgg.Text)
                objcommand.Parameters.AddWithValue("@标识卡号", bskh.Text)
                objcommand.Parameters.AddWithValue("@产品编号", cpbh.Text)
                objcommand.Parameters.AddWithValue("@用户名称", yhmc.Text)
                objcommand.Parameters.AddWithValue("@所属公司", butssgs.Text)
                objconn.Open()
                objcommand.ExecuteNonQuery()
                objconn.Close()
                MessageBox.Show("资料录入保存成功", "保存成功", MessageBoxButtons.OK, MessageBoxIcon.Information, MessageBoxDefaultButton.Button1)
                Me.jqxh.Text = ""
                Me.pmgg.Text = ""
                Me.bskh.Text = ""
                Me.cpbh.Text = ""
                Me.yhmc.Text = ""
                Me.jqxh.Enabled = False
                Me.pmgg.Enabled = False
                Me.bskh.Enabled = False
                Me.cpbh.Enabled = False
                Me.yhmc.Enabled = False
                Me.ButAdd.Enabled = True
                Me.butSave.Enabled = False
                Me.cpbh.Text = ""
            End If

        Else
            MessageBox.Show("录入项不能为空，请填写完整", "添加提示", MessageBoxButtons.OK, MessageBoxIcon.Asterisk, MessageBoxDefaultButton.Button1)

        End If
        



    End Sub

    Private Sub butcll_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Butcll.Click
        addbut = False
        Me.jqxh.Text = ""
        Me.pmgg.Text = ""
        Me.bskh.Text = ""
        Me.cpbh.Text = ""
        Me.yhmc.Text = ""
        Me.jqxh.Enabled = False
        Me.pmgg.Enabled = False
        Me.bskh.Enabled = False
        Me.cpbh.Enabled = False
        Me.yhmc.Enabled = False
        Me.ButAdd.Enabled = True
        Me.butSave.Enabled = False
        Me.Butcll.Enabled = False
    End Sub

   
    Private Sub ButAdd_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButAdd.Click
        addbut = True
        Me.bskh.Text = ""
        Me.cpbh.Text = ""
        Me.yhmc.Text = ""
        Me.jqxh.Enabled = True
        Me.pmgg.Enabled = True
        Me.bskh.Enabled = True
        Me.cpbh.Enabled = True
        Me.yhmc.Enabled = True
        Me.ButAdd.Enabled = False
        Me.butSave.Enabled = True
        Me.Butcll.Enabled = True
    End Sub

    Private Sub Butclose_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Butclose.Click
        addbut = False
        Me.jqxh.Text = ""
        Me.pmgg.Text = ""
        Me.bskh.Text = ""
        Me.cpbh.Text = ""
        Me.yhmc.Text = ""
        Me.jqxh.Enabled = True
        Me.pmgg.Enabled = True
        Me.bskh.Enabled = True
        Me.cpbh.Enabled = True
        Me.yhmc.Enabled = True
        Me.ButAdd.Enabled = False
        Me.butSave.Enabled = True
        Me.Close()
    End Sub

    Private Sub bskh_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles bskh.TextChanged

    End Sub

    Private Sub zllrform_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'TODO: 这行代码将数据加载到表“家电下乡资料DataSet1.公司名称”中。您可以根据需要移动或移除它。
        Me.公司名称TableAdapter.Fill(Me.家电下乡资料DataSet1.公司名称)
        'TODO: 这行代码将数据加载到表“家电下乡资料DataSet1.品名规格分表”中。您可以根据需要移动或移除它。
        Me.品名规格分表TableAdapter.Fill(Me.家电下乡资料DataSet1.品名规格分表)
        'TODO: 这行代码将数据加载到表“家电下乡资料DataSet1.机器型号分表”中。您可以根据需要移动或移除它。
        Me.机器型号分表TableAdapter.Fill(Me.家电下乡资料DataSet1.机器型号分表)
        'TODO: 这行代码将数据加载到表“家电下乡资料DataSet1.家电下乡资料”中。您可以根据需要移动或移除它。
    End Sub

    Private Sub 所属公司Label_Click(ByVal sender As System.Object, ByVal e As System.EventArgs)

    End Sub
End Class



[ 本帖最后由 卓越电脑 于 2010-8-16 20:32 编辑 ]

标识卡号是不是主键？
如果是，那么在插入新记录之前先检查一下数据库里有没有这个标识卡号，有了就不插入，没有就插入。

我是这样做的 cn.Close()
        cn.Open()

        '顺序读数据库的用户表，判断是否出现重名或者用户名为空
        Dim myCmd As New OleDb.OleDbCommand("select * from 用户表", cn)
        Dim dr As OleDbDataReader = myCmd.ExecuteReader()
        While dr.Read()
            If dr.Item(0) = Me.用户名TextBox.Text Or Me.用户名TextBox.Text = "" Then
                Beep()
                MsgBox("用户名已经存在或者为空！", , "请输入新的用户名")
                用户名TextBox.Text = ""
                TxtCode.Text = ""
                TxtCfmCode.Text = ""
                用户名TextBox.Focus()
                i = 1
                Exit While
            End If
        End While
        dr.Close()

可以直接用个命令读出数据库的数据吗？我是放到一个datareader里面再读出的……

已解决，谢谢大家的帮助
Dim connstr As String
            Dim providername As SqlClient.SqlConnection
            connstr = "server=localhost;database=家电下乡资料;user id=sa;password=xzzydnwyc"
            providername = New System.Data.SqlClient.SqlConnection(connstr)
            providername.Open()
            Dim sqlcmd As SqlClient.SqlCommand
            Dim sqlcmdstr As String
            sqlcmdstr = "select * from 家电下乡资料 " & " where 标识卡号='" & bskh.Text & "'"
            sqlcmd = New SqlClient.SqlCommand(sqlcmdstr, providername)
            Dim rd As SqlClient.SqlDataReader
            rd = sqlcmd.ExecuteReader()
            If rd.Read() Then
                MessageBox.Show("此卡已录入，不能重复录入", "添加提示", MessageBoxButtons.OK, MessageBoxIcon.Asterisk, MessageBoxDefaultButton.Button1)
                bskh.Text = ""
                cpbh.Text = ""
                bskh.Focus()
                cpbh.Focus()
            Else