problem with clearing the datagridview

[atzdgreat]

i made if statement in my programs that allow the user to choose days...

    Private Sub cmdsummaryview_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdsummaryview.Click

        ClearGridView() '----> this stands for Clearing the Datagrid


            If NewEveryCount.ToString = 1 Then
                SQL = "SELECT `idnumber`, `employeename`, `rot1`, `ot1` FROM `employeewithperiod` WHERE `mdclientperiod`='" + MD5CalcString(cmbclient.Text + "-" + txtcperiod.Text) + "' ORDER BY employeename ASC"
            ElseIf NewEveryCount.ToString = 2 Then
                SQL = "SELECT `idnumber`, `employeename`, `rot2`, `ot2` FROM `employeewithperiod` WHERE `mdclientperiod`='" + MD5CalcString(cmbclient.Text + "-" + txtcperiod.Text) + "' ORDER BY employeename ASC"
            endif

        myCommand.CommandText = SQL
        myCommand.Connection = MySQLConnectionString
        myAdapter.SelectCommand = myCommand
        myBuilder = New MySql.Data.MySqlClient.MySqlCommandBuilder(myAdapter)
        myAdapter.Fill(myDataSet, "employeewithperiod")
        myBindingSource.DataSource = myDataSet.Tables("employeewithperiod")
        DataGridView1.DataSource = myBindingSource
        DataGridView1.Refresh()

End Sub

    Public Sub ClearGridView()
        myDataSet.Clear()
        myDataSet.Dispose()
        myBindingSource.DataSource = myDataSet
        DataGridView1.DataSource = myBindingSource
        'DataGridView1.DataSource = myDataSet
        DataGridView1.DataSource = Nothing
    End Sub

at first if my NewEveryCount.ToString is = 1, it will successfully load what field i want to display in my datagridview (that is rot1, ot1 and etc). but when im going to click the button again and change change my NewEveryCount.ToString = 2. it will just add another column for rot2, ot2 and so on and so fort. in short my column will idnumber employeename rot1 ot1 rot2 ot2

hmmm i dont know what's wrong with my datagrid.. that's why im thinking if im going to leave my datagrid and move to listview.. but i want to explore datagridview.. hope any one can give me a hand for this.. thanks guys