我正在努力获取这两个查询：

SELECT min(to_date(nullif(mydatetext,''), 'DD.MM.YYYY')) AS dmin,
       max(to_date(nullif(mydatetxt,''), 'DD.MM.YYYY')) AS dmax
FROM mytable

和

SELECT count(*)
FROM mytable
WHERE
to_date(nullif(mydatetxt,''))  'ERROR HERE
BETWEEN 
max(to_date(nullif(mydatetxt,''), 'DD.MM.YYYY'))
AND
min(to_date(nullif(mydatetxt,''), 'DD.MM.YYYY'))

我可以将结果读取为最小日期，最大日期，最小和最大日期之间（包括最小和最大日期）的日期计数。但是这里有几个问题。

第二个查询无法按预期运行或根本无法运行，因此必须进行改进。如果可以在单个查询（？）中编写这两个查询，我可以将第一部分中的dmin和dmax变量用作第二部分中的变量吗？像这样：

SELECT count(*)
FROM mytable
WHERE
to_date(nullif(mydatetxt,''))  'ERROR HERE
BETWEEN 
dmin
AND
dmax

最后请帮助解决这种情况。

可行的代码：

Using cmd As New NpgsqlCommand("SELECT my_id, mydate FROM " & mytable, conn)
Using dr As NpgsqlDataReader = cmd.ExecuteReader()
    While dr.Read()
        mydate = CStr(dr(1))

        If IsDate(mydate) Then
            Dim dat As Date = CDate(mydate.Substring(6, 4) & "/" & mydate.Substring(3, 2) & "/" & mydate.Substring(0, 2))
            If dat < mindate Or mindate = Nothing Then
                mindate = dat
            End If

            If dat > maxddate Or maxdate = Nothing Then
                maxdate = dat
            End If

            count += 1
        End If
    End While
End Using
End Using

解决方案：最终这是非常快的改进版本，欧文（Ervin）请给：

        Using cmd As New NpgsqlCommand( _
             "WITH base AS (" & _
             "  SELECT TO_DATE(datum, 'DD.MM.YYYY') AS the_date " & _
             "  FROM " & myKalkTable & " " & _
             "  WHERE datum <> '') " & _
             "  SELECT MIN(the_date) AS dmin, " & _
             "         MAX(the_date) AS dmax, " & _
             "         COUNT(*) AS ct_incl, " & _
             "  (SELECT COUNT(*) " & _
             "         FROM base b1 " & _
             "         WHERE(b1.the_date < max(b.the_date)) " & _
             "         AND b1.the_date > min(b.the_date)) " & _
             "         AS ct_excl " & _
             "         FROM base b", conn)

            Using dr As NpgsqlDataReader = cmd.ExecuteReader()
                While dr.Read()
                    mindate = CType(CDate(CStr(dr(0))), Date)
                    maxdate = CType(CDate(CStr(dr(1))), Date)
                    count = CInt(dr(2))
                End While
            End Using
        End Using