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我的 Visual Basic 程序从 Access 数据库中获取值并将它们放在标签中。从数据库返回的数据取决于在组合框中选择的值，并且应该随着组合框值的变化而变化，但令我惊讶的是，即使它们必须更改，标签仍然只显示最先显示的第一个值。我试图在每次访问数据库之前将标签文本分配给任何内容，但它仍然显示以前的值。我无法让这些标签更改文本可能是什么问题？请帮忙。这是我用来访问数据库并放入标签的代码：

    Me.p.Text = Nothing
    Me.tq.Text = Nothing
    Me.pd.Text = Nothing
    Me.ba.Text = Nothing
    Me.o.Text = Nothing
    Me.pm.Text = Nothing
    Me.tv.Text = Nothing
    Me.tqr.Text = Nothing
    Me.dff.Text = Nothing
    Try
        cmd.Connection = cnn
        cmd2.Connection = cnn
        cmd3.Connection = cnn
        cmd.CommandText = "SELECT distinct(brought_price), sum(brought_qtty), sum(brought_paid), sum(brought_bal),recieved_qtty,sum(brought_qtty)-recieved_qtty, sum(brought_qtty)*brought_price from brought_coffee, farmer, centre_weigh " & _
                                "where brought_date=? and farmer_centre=? and" & _
                                " brought_coffee.farmer_num=farmer.farmer_num and weigh_date=? and weigh_centre=? and brought_coffee.brought_date=centre_weigh.weigh_date and farmer_centre=weigh_centre group by brought_price,recieved_qtty"
            MessageBox.Show(Me.dtc.Text)
            cmd.Parameters.Add("@dt", OleDb.OleDbType.Date).Value = Me.dtc.Text
            cmd.Parameters.Add("@dt2", OleDb.OleDbType.VarChar).Value = Me.cc.Text.Trim.Substring(0, 2)
            cmd.Parameters.Add("@dt3", OleDb.OleDbType.Date).Value = Me.dtc.Text
            cmd.Parameters.Add("@dt4", OleDb.OleDbType.VarChar).Value = Me.cc.Text.Trim.Substring(0, 2)

            cmd2.CommandText = "SELECT sum(brought_qtty) from brought_coffee,farmer where coffee_grade='O' and brought_date=? and farmer_centre=? and" & _
                                " brought_coffee.farmer_num=farmer.farmer_num"
            cmd2.Parameters.Add("@dt5", OleDb.OleDbType.Date).Value = Me.dtc.Text
            cmd2.Parameters.Add("@dt6", OleDb.OleDbType.VarChar).Value =     Me.cc.Text.Trim.Substring(0, 2)

            cmd3.CommandText = "SELECT sum(brought_qtty) from brought_coffee,farmer where coffee_grade='P' and brought_date=? and farmer_centre=? and" & _
                                " brought_coffee.farmer_num=farmer.farmer_num"
            cmd3.Parameters.Add("@dt7", OleDb.OleDbType.Date).Value = Me.dtc.Text
            cmd3.Parameters.Add("@dt8", OleDb.OleDbType.VarChar).Value = Me.cc.Text.Trim.Substring(0, 2)

            myData = cmd.ExecuteReader
            myData2 = cmd2.ExecuteReader
            myData3 = cmd3.ExecuteReader
            'MessageBox.Show("check 1")

            While myData.Read()
                'MessageBox.Show("check 1")
                Dim pr As String = myData(0).ToString
                Dim qtty As String = myData(1).ToString
                Dim pd As String = myData(2).ToString
                Dim bl As String = myData(3).ToString
                Dim rq As String = myData(4).ToString
                Dim dq As String = myData(5).ToString
                Dim tvp As String = myData(6).ToString
                'MessageBox.Show("quantity" & pr)
                Me.p.Text = pr
                Me.tq.Text = qtty
                Me.pd.Text = pd
                Me.ba.Text = bl
                Me.tqr.Text = rq
                Me.dff.Text = dq
                Me.tv.Text = tvp

            End While
            myData.Close()

            While myData2.Read()
                Dim od As String = myData2(0).ToString
                'MessageBox.Show(od)
                Me.o.Text = od
            End While

            myData2.Close()

            While myData3.Read()
                Dim pmm As String = myData3(0).ToString
                Me.pm.Text = pmm
            End While
            myData3.Close()
            cnn.Close()

dtc并且cc是与选定的数据库值进行比较的文本框

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