Aggregation using CASE I have a table called country:

CountryName | Sales
A             200
B             100
C             300
D             500
E             400


Now I in my report I have hardcoded the parameter of region that the user will select, such as RegionEast, RegionWest etc. 

My requirement is that if I select RegionWest then I should aggregate countries (A, C). If (B, D) then RegionEast. There is no explicit relationship of a region with a country in the database.

I was trying to write my query like this:

Case When Parameter ='RegionWest' Then Country in (A,C)
     when Parameter ='RegionEast' Then Country in (B,D)
 Else Country


But that is not working as expected. Can somebody help?
The CASE you wrote is for visualization (SELECT) or for filter data (WHERE).

if is the SELECT case, then can be (not sure if you want an SUM(Sales):

SELECT
  (CASE WHEN Parameter = 'RegionWest' THEN 'A,C'
        WHEN Parameter = 'RegionEast' THEN 'B,D'
  ELSE Country) AS CountryName, Sales
FROM yourTable


If is the WHERE case I need more of your SQL to understand your question.