Descendants based on Prompt

[okh310]

Hello!  I have a crosstab report where I want to display "column A" and "column B".

Column A is derived from user selection from a tree prompt. 
So code looks something like this:  set (#promptmany ('CostCat','MUN','[Cost Breakdowns].[Cost Categories].[Cost Categories].[Total Cost Categories]->:[TM].[Cost Categories].[Cost Categories].[@MEMBER].[Grand Total]')#)

Column B:
descendants(currentMember(hierarchy([Cost Category Level 1])),1)

This works fine if user selects anything but the lowest level.  When they choose the lowest level in the tree prompt, I get no data found because there are no descendants to display.  If user selects lowest level, I basically want to just display that level in Column B.  So Column A and B should be the same.

How can I implement this? Thank you in advance.

[CognosPaul]

Interesting issue. The descendants function doesn't seem to work well with dynamic values, so the expected
descendants(
  set()
  , if(roleValue('_levelNumber',currentMember()= 4) then (0) else (1)
)

doesn't work.

You can't return sets in an IF statement, so if(levelNumber=4) then (set()) else (set()) won't work.

The only solution I've found is a bit clunky:

  union(
    children(currentMember([sales_and_marketing].[Time].[Time]))
    ,head(
currentMember([sales_and_marketing].[Time].[Time])
  ,if(count(1 within set children(currentMember([sales_and_marketing].[Time].[Time])))=0)
    then (1)
    else (0)
  )

  )
)

It takes the children of the current member, and unions either the currentMember if there are no children, or an empty set if there are.

[okh310]

Thank You Paul!!!!

This worked quite nicely.  It's a bummer you can't use if then statements for sets.  You are right, descendants function does not work very well dynamically.