Sql 查询不包括指定的函数

我正在尝试在MS Access 2010数据库中运行以下查询：

SELECT a.[Level], max(a.dte) AS nextDate, IIf(a.[Type1Date]<a.[Type2Date],"t1","t2") AS Type
FROM (
    select [Level], Type1Date as dte, Type1Date, Type2Date 
    FROM CommunicationTable WHERE ClientNumber=1
    UNION
    select [Level], Type2Date as dte, Type1Date, Type2Date 
    FROM CommunicationTable WHERE ClientNumber = 1
)  AS a
GROUP BY a.[Level];  

---

选择[Level]、max（a.dte）作为下一个日期、IIf（a[Type1Date]使用组查询时，不能在SELECT语句中包含列，除非它们也在group By语句或集合中。因此，您可能希望使用：

GROUP BY a.[Level], IIf(a.[Type1Date]<a.[Type2Date],"t1","t2")

GROUP BY a.[Level]、IIf（a.[Type1Date]通过组查询，您不能在Select语句中包含列，除非它们也在GROUP BY语句或集合中。因此，您可能希望使用：

GROUP BY a.[Level], IIf(a.[Type1Date]<a.[Type2Date],"t1","t2")

按[Level]，IIf（a.[Type1Date]分组