MDX - Top N elements of every subgroup - ssas

Question

Let's assume I have a cube with a sales fact table and 3 dimensions: Time (with hierarchy year-month-day), Geography (with hierarchy continent-country-region) and Product with some characteristics about the product sold, let's take the brand for instance.
What I am trying to do is to display the top N brands with respect to the measure chosen, in this case average sales (which is already in the cube as total sales/number of products sold), for every region and month.
I also need to display the country and the year, for clearness.
I have searched for the solution to this everywhere, I came close to it but not completely. I hope someone can help me figure it out.
So I used generate and topcount with the following query, but the problem is that the topcount calculates the N best selling brands over the whole dataset, not for every region and month subgroup. And then applies this top N to every subgroup.
WITH SET [Top N Brands] AS
Generate([Geography].[Region].Children,
TopCount(Geography].[Region].CurrentMember * [Gpu Product].[Brand].[Brand].MEMBERS,
5, ([Measures].[averageSales])))
SELECT {[Measures].[averageSales]} ON COLUMNS,
NON EMPTY ( [Time].[Year].[Year],
[Time].[Month].[Month],
[Geography].[Country].[Country],
[Top N Brands]) ON ROWS
FROM [Cube]
So I am getting this, with the global top 5 brands distributed over the regions, if sold there:
But I should get this with different top 5s for every region:
What am I missing?

Answer 1

You need to use rank. Take a look at the example below. I am using the sample Adventure Works Db, here I am listing For each country, for each product category in that country, the top three subcategories according to internet sales.
WITH
MEMBER [Measures].[Internet Sales Amount Rank] AS
RANK( ([Customer].[Country].currentmember,[Product].[Category].currentmember,[Product].[Subcategory].CurrentMember),
ORDER( ([Customer].[Country].currentmember,[Product].[Category].currentmember,[Product].[Subcategory].[Subcategory].Members) , [Measures].[Internet Sales Amount], BDESC)
)
select
non empty
([Measures].[Internet Sales Amount])
on columns
,
non empty
([Customer].[Country].[Country],
[Product].[Category].[Category],
filter([Product].[Subcategory].[Subcategory],[Measures].[Internet Sales Amount Rank]<4))
on rows
from [Adventure Works]
Result