DataTables("税务销售").LoadFilter = "单据编码 Is Null"
DataTables("税务销售").Load()

Tables("税务销售").Sort = "税务年度,销方名称 DESC"
Dim t1 As Table = Tables("税务销售")
Dim cn As Integer = t1.Rows.Count - 1
Dim k As Integer = 1
Dim s As Row
For i As Integer = 0 To cn
    If t1.Rows(i)("开票日期") = Nothing Then
        t1.Rows(i)("单据编码") = Nothing
    Else
        Dim d As Date = t1.Rows(i)("开票日期")
        Dim y As Integer = d.Year
        Dim m As Integer = d.Month
        Dim Days As Integer = Date.DaysInMonth(y, m)
        Dim fd As Date = New Date(y, m, 1) \'获得该月的第一天
        Dim ld As Date = New Date(y, m, Days) \'获得该月的最后一天
       
        Dim bh As String = Format(t1.Rows(i)("开票日期"), "yyyyMM") \'取得编号的6位前缀
        \'If t1.Rows(i)("单据编码").StartsWith(bh) = False Then\'如果编号的前6位不符
            Dim max As String
            Dim idx As Integer
            max = DataTables("税务销售").sqlCompute("Max(单据编码)", "开票日期 >= #" & fd & "# And 开票日期 <= #" & ld & "# And [_Identify] <> " & t1.Rows(i)("_Identify")) \'取得该月的最大编号
            MessageBox.Show(max)
            If max > "" Then \'如果存在最大编号
                idx = CInt(max.Substring(7, 3)) + 1 \'获得最大编号的后三位顺序号,并加1
            Else
                idx = 1 \'否则顺序号等于1
            End If
            If t1.Rows(i)("购买方名称") <> t1.Rows(i - 1)("购买方名称") Then
                t1.Rows(i)("单据编码") = bh & Format(idx, "000")
            Else
                t1.Rows(i)("单据编码") = t1.Rows(i - 1)("单据编码")
            End If
        \'End If
    End If
Next
 MessageBox.Show("已完成")

下边求不出最大的编号，弹出窗口为空值

max = DataTables("税务销售").sqlCompute("Max(单据编码)", "开票日期 >= #" & fd & "# And 开票日期 <= #" & ld & "# And [_Identify] <> " & t1.Rows(i)("_Identify")) \'取得该月的最大编号
            MessageBox.Show(max)

解决了