主题：[求助如何计算上月的结存数量

Select Case e.DataCol.Name
    Case "月份"
        For Each dr As DataRow In DataTables("仓库存在统计表").DataRows
            Dim dt As DataTable = DataTables("仓库入货明细")
            Dim Filter As String = "类别 = '" & dr("类别") & "' And "
            Filter = Filter & "品名 = '" & dr("品名") & "' And "
            Filter = Filter & "规格与色号 = '" & dr("规格与色号") & "' And "
            Filter = Filter  & "单位 = '" & dr("单位") & "' And "
            Filter = Filter & "单价 = " & dr("单价")
            dr("本月_入库_数量") = dt.Compute("Sum(入库数量)",Filter & "And [入库月份] ='" & e.DataRow("月份") & "'")
            dr("本月_出库_数量") = dt.Compute("Sum(出库数量)",Filter & "And [出库月份] ='" & e.DataRow("月份") & "'")

            ’这个代码是计算本月的出入库情况

            dr（"上月结存_数量")=?   这个不会,举例我本月计算为5月份,我要计算4月份的结存数量,怎么写?
      Next
End Select