使用子查询进行分组时出现SQL Server 2005错误

Using SQL Server 2005 I'm trying to group based on a case statement with a subquery, but I'm getting an error ("Each GROUP BY expression must contain at least one column reference. "). I can work round it quite easily, but can anyone explain the error? I've got a column reference to #header.header.Using SQL Server 2005 I'm trying to group based