Cases on SQL Query

Question

I want to apply various cases on my query according to functionality.
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?