查询不包括特定的功能

Question

我试图运行在一个MS Access 2010数据库以下查询：

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]; 

但是，Access是给我一个对话框，指出：

You tried to execute a query that does not include the specified 
expression 'IIf(a.[Type1Date]<a.[Type2Date],"t1","t2")' as part 
of an aggregate function. 

灿任何人都可以解释这是什么意思，并告诉我如何修复代码，以便它返回我请求的字段（Level，nextDate，Type）？

Answer 1

对于组查询，不能在Select语句中包含列，除非它们也在Group By语句或聚合中。所以你可能想要使用：

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