Filter in WHERE using Named Set - ssas

Question

Is it possible to use an aggregated named set to filter queries in MDX? I don't want the set items in the result set so moved it to the WHERE, however this seems to cause all measures to return (null).
First, I build a set using a couple of members from a staff hierarchy:
WITH
SET [Combined] as {
[Staff].[Group].[Practice Group].&[04],
[Staff].[Group].[Practice Group].&[06]
}
Then I aggregate that set into a new member:
MEMBER [Staff].[Group].[Group Combo] AS Aggregate([Combined])
Usually I would then use this in my query as a reporting area, possibly with hierarchize (depending on the query) and everything is fine. However this time I needed to filter the data based on this combination of members e.g:
WHERE ([Staff].[Group].[Group Combo])
This gave me (null) values and if I used [Combined] I received a cyclical reference error, however using the below worked fine:
WHERE ({[Staff].[Group].[Practice Group].&[04],[Staff].[Group].[Practice Group].&[06]})
Am I overlooking something here? Or using the wrong approach? Maybe it is just a quirk of the cube I am querying?

Answer 1

I would say that the behaviour is related to the order in which the processor is executing the clauses within your query:
Logical order an MDX query is processed
WHERE happens before WITH
You mentioned that the following works fine - specifying explicit members in the where clause it is pretty standard and fast:
WHERE ({[Staff].[Group].[Practice Group].&[04],[Staff].[Group].[Practice Group].&[06]})
You could add it to a sub-select just as it is on the 0 axis:
SELECT
...
...
FROM
(
SELECT
{[Staff].[Group].[Practice Group].&[04],[Staff].[Group].[Practice Group].&[06]} ON 0
FROM [YourCube]
);