Multiple Cases in SQL query

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SQL-Server-2008R2

There is a SQL query given below, that query performs DateDIff operation by slecting max and min date according to Card ar you can say id,
here' the query

SELECT
--COUNT(DIFF)
Count(DATEDIFF(D, MinDate, MaxDate)/30) AS DIFF

FROM (
SELECT

   MAX(TDate) AS MaxDate,
   MIN(TDate) AS MinDate
   FROM EDATA
   GROUP BY TCard
)a
Group by DATEDIFF(D, MinDate, MaxDate)/30

Now i want to apply some kind of sorting in this query before performing DateDiff operation (sorting can be based on several parameters like age, name etc. but 1 at a time according to the cases)
Now I want to know How to apply cases over this query an where?