主题：跨表数据引用和计算

为什么我在答题数据1写入下列代码后，成绩表中成绩1列只有主观题2得相应分值呢？

Dim zgt As Integer    '声明储值变量
Dim zgt1 As Integer    '声明储值变量
Dim zgt2 As Integer    '声明储值变量
Dim Filter As String = "学号 = '" & e.DataRow("学号") & "'"  '声明关联列
Dim drs As List(of DataRow) = DataTables("成绩表").Select(Filter)  '声明关联列变量集合
Dim dr As DataRow '搜索变量集合每一个成员
Select Case e.DataCol.Name
    Case "主观题1"   '确定主表改变列
        For Each dr In drs
            If e.NewValue = "t" Then
                zgt1 = 1
            End If
            If e.NewValue = "f" Then
                zgt1 = 0
            End If      '给相关列赋值
        Next
    Case "主观题2"   '确定主表改变列
        For Each dr In drs
            If e.NewValue = "t" Then
                zgt2 = 1
            End If
            If e.NewValue = "f" Then
                zgt2 = 0
            End If      '给相关列赋值
        Next
        zgt=zgt1+zgt2
        dr("成绩1") = zgt   '将相关值写入从表相关列

end select