Listbox filtering based on radiolist sql statement error

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ASP.NET

got a huge problem here. I know that my query is not complete but i dont know what the exact function that i am looking for. As you see ive created a radiobuttonlist A-Z and i want my list box to be filtered according to what the selected value of my radiolist. My problem is that when I clicked a letter all the data composing of choosed letter a will be displayed on my list box where i just want the first letter of the data to be published. Sorry for my english and more power
note that filtering is the name of my radiolist

Public Sub listcoursecode()
        Do While strsql = String.Empty
            strsql = "select * from [tblCourseTitle] where CourseCode like '%" & Trim(filtering.SelectedValue) & "%' Order by [CourseCode]Asc"
            Dim cmd As SqlCommand = New SqlCommand(strsql, cn)
            cn.Open()
            lbCourseCode.DataSource = cmd.ExecuteReader
            lbCourseCode.DataTextField = "CourseCode"
            lbCourseCode.DataValueField = "CourseTitle"
            lbCourseCode.DataBind()
        Loop
        cn.Close()
        strsql = String.Empty
    End Sub

    Protected Sub filtering_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles filtering.SelectedIndexChanged
        listcoursecode()
    End Sub
End Class