主题：查询按钮代码求助？

DataTables("鉴定上报表").DataRows.Clear
For Each jdgz As String In dt.GetValues("鉴定工种")
    Dim dr As DataRow = DataTables("鉴定上报表").AddNew
    dr("鉴定工种") = jdgz
    dt = DataTables("报名登记表")
    dr("鉴定考核人数_初级工_2") = dt.Compute("Count(身份证号码)", "鉴定等级 = '初级工' and 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("鉴定考核人数_中级工_3") = dt.Compute("Count(身份证号码)", "鉴定等级 = '中级工' and 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("鉴定考核人数_高级工_4") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级工' And 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("鉴定考核人数_技师_5") = dt.Compute("Count(身份证号码)", "鉴定等级 = '技师' and 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("鉴定考核人数_高级技师_6") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级技师' And 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dt = DataTables("证书编号表")
    dr("取得证书人数_初级工_8") = dt.Compute("Count(身份证号码)", "鉴定等级 = '初级工' and 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("取得证书人数_中级工_9") = dt.Compute("Count(身份证号码)", "鉴定等级 = '中级工' and 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("取得证书人数_高级工_10") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级工' And 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("取得证书人数_技师_11") = dt.Compute("Count(身份证号码)", "鉴定等级 = '技师' and 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
    dr("取得证书人数_高级技师_12") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级技师' And 鉴定工种 = '" & dr("鉴定工种")  & "' and " & filter )
   
Next

DataTables("鉴定上报表").DataRows.Clear
For Each dm As String In dt.GetValues("代码")
    Dim dr As DataRow = DataTables("鉴定上报表").AddNew
dt = DataTables("报名登记表")
    dr("鉴定考核人数_初级工_2") = dt.Compute("Count(身份证号码)", "鉴定等级 = '初级工' and 代码 = '" & dr("代码")  & "' and " & filter )
    dr("鉴定考核人数_中级工_3") = dt.Compute("Count(身份证号码)", "鉴定等级 = '中级工' and 代码 = '" & dr("代码")  & "' and " & filter )
    dr("鉴定考核人数_高级工_4") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级工' And 代码 = '" & dr("代码")  & "' and " & filter )
    dr("鉴定考核人数_技师_5") = dt.Compute("Count(身份证号码)", "鉴定等级 = '技师' and 代码 = '" & dr("代码")  & "' and " & filter )
    dr("鉴定考核人数_高级技师_6") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级技师' And 代码 = '" & dr("代码")  & "' and " & filter )
    dr("代码") = dm
    dt = DataTables("证书编号表")
    dr("取得证书人数_初级工_8") = dt.Compute("Count(身份证号码)", "鉴定等级 = '初级工' and 代码 = '" & dr("代码")  & "' and " & filter )
    dr("取得证书人数_中级工_9") = dt.Compute("Count(身份证号码)", "鉴定等级 = '中级工' and 代码 = '" & dr("代码")  & "' and " & filter )
    dr("取得证书人数_高级工_10") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级工' And 代码 = '" & dr("代码")  & "' and " & filter )
    dr("取得证书人数_技师_11") = dt.Compute("Count(身份证号码)", "鉴定等级 = '技师' and 代码 = '" & dr("代码")  & "' and " & filter )
    dr("取得证书人数_高级技师_12") = dt.Compute("Count(身份证号码)", "鉴定等级 = '高级技师' And 代码 = '" & dr("代码")  & "' and " & filter )
   
Next

上述代码的结果：“代码”列有数据，但“鉴定工种”列没有数据，为空，这是什么原因？