将CASE语句设置为变量，然后在GROUP BY子句中使用该变量

I am using SQL Server 2008 and have a very large CASE statement that is also used in the GROUP By clause. I would like to set the CASE statement to a variable in order to minimize code maintenance and maximize reuse. The problem is that I get this error:I am using SQL Server 2008 and have a very larg