主题：计算及格率、优秀率时的空值

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在研究FT的经典例子中，遇到一个问题，计算及格率、优秀率时，要是“化学”学科没有考试，即“成绩表”中“化学”列的数值是空值，按照下列的代码计算的及格率，优秀率全部出现0.00%，这种情况怎么处理呢，能不能让没有考试的“化学”及格率、优秀率为空值呢？

Dim b As New SQLGroupTableBuilder("统计表1","成绩表")
b.Groups.AddDef("班级")
b.Totals.AddExp("人数","学号","",AggregateEnum.Count) '统计人数
Dim nms() As String = {"语文","英语","数学","物理","化学"}
For Each nm As String In nms '利用表达式统计及格人数和优秀人数
    b.Totals.Addexp(nm & "_及格", "iif(" &  nm & " >= 60, 1, 0)")
    b.Totals.Addexp(nm & "_优秀", "iif(" &  nm & " >= 90, 1, 0)")
Next
b.VerticalTotal = True
b.Build()
For Each nm As String In nms '对生成的统计表增加表达式列,计算及格率和优秀率
    DataTables("统计表1").DataCols.Add(nm & "_及格率",Gettype(Double), nm & "_及格/人数" )
    Tables("统计表1").Cols(nm & "_及格率").Move(Tables("统计表1").Cols(nm & "_及格").Index + 1)
    DataTables("统计表1").DataCols.Add(nm & "_优秀率",Gettype(Double), nm & "_优秀/人数" )
    Tables("统计表1").Cols(nm & "_优秀率").Move(Tables("统计表1").Cols(nm & "_优秀").Index + 1)
    DataTables("统计表1").DataCols(nm & "_及格率").SetFormat("0.00%")
    DataTables("统计表1").DataCols(nm & "_优秀率").SetFormat("0.00%")
Next
MainTable = Tables("统计表1")

修改公式，如

DataTables("统计表1").DataCols.Add(nm & "_及格率",Gettype(Double), "iif(" & nm & "_及格 is null, null, " & nm & "_及格/人数)" )