比如表 P_History数据库（Access)的数据如下：

C_ID        M_ID    P_ID
20120102    01001    0
20120103    01001    0
20120104    01001    1
20120105    01003    1
20120106    02001    0
20120107    02001    2
20120108    02001    0
20120109    02001    1
20120110    01003    2

P_ID中:0表示Ok,1表示Fail,2表示,UnKnow，如何显示出这样的结果呢：

C_ID    M_ID    P_ID
20120102    01001    ok
20120103    01001    ok
20120104    01001    Fail
20120105    01003    Fail
20120106    02001    ok
20120107    02001    Unknow
20120108    02001    ok
20120109    02001    Fail
20120110    01003    Unknow

自定义函数:

Function Str_PM_Result(ByVal str_P As String) As String
If str_P = "0" Then
    Str_PM_Result = "For Schedule PM"
ElseIf str_P = "1" Then
    Str_PM_Result = "For Down PM"
Else
    Str_PM_Result = "For Schedule PM"
End If
End Function

sql = "select C_ID,M_ID," & Str_PM_Result(P_ID) & " from P_History"

如上语句，但是P_ID栏目为空，并不是 Ok/Fail/Unknow"?
或者有时会出现错误:
出现错误:  -2147217900    语法错误(操作符丢失)正查询表达式 'For Schedule PM' 中

或者:

SELECT IIF(P_Id=0,'OK',IIF(P_ID=1,'fail','unknow')) AS P_Id FROM 

但是 P_ID栏目还是为空，不知道哪里出错了...