Grouping Tasks in Reports(CP 2012 SP2)

QUESTION:
Is there a way to group sub tasks under their parent task for reporting purposes? For example:

-Parent Task
 - Sub Task 1
 - Sub Task 2

What field would allow me to group the tasks like this in a report?

ANSWER:
Parsing through the OBS would be about the only way to group the sub tasks in Report Designer, but it would have to have a finite number of levels.
Maybe something like the immediate parent task (ParentTaskID) or the upper-most parent task (based on where the taskid is in the OBS). It's tricky and could have performance issues.
Cognos may be a bit more flexible since it can call stored procedures or functions, but it would be basically along the same lines.

ADDITIONAL NOTES:
The « ParentTaskID » would be a custom field in the data collection.
To define it, would require you to create a function that would return the parent of a taskID. The function would use the WBS field for this purpose.