1、Group By 语句中：select指定的字段必须是“分组依据字段”，其他字段若想出现在select中则必须包含在聚合函数中。
例如：
select [col1], avg([col2]) from [tb] group by [col1];
2、Order By和Group By 一起使用时需要注意： ORDER BY 子句中的列必须包含在聚合函数或 GROUP BY 子句中。
例如：SELECT [col1] ,[col2],MAX([col3]) FROM [tb] GROUP BY [col1] ,[col2] ORDER BY [col1] ,[col2] ,MAX([col3])SELECT [col1] ,[col2],MAX([col3]) AS [col3] FROM [tb] GROUP BY [col1] ,[col2] ORDER BY [col1] ,[col2] ,[col3]SELECT [col1] ,[col2] FROM [tb] GROUP BY [col1] ,[col2] ,[col3] ORDER BY [col1] ,[col2] ,[col3]
原文：http://www.cnblogs.com/xubiao/p/5563916.html