Not sure I understand what exactly is the question... Are you building a query (SQ01) or an ABAP program? For the query you just need to know what the relationship between the tables is (when the key fields are obvious, they would even be proposed automatically when you try to do JOIN) and how the records are stored.
There are tools in SAP to do this kind of analysis - e.g. ST05 (SQL trace) and SE16 (Data viewer).
Also you might want to change the post subject to be more specific, so that the SCN members who know more about the specifics of PS/COPA reporting would see this thread and reply. Or maybe this should actually be posted in PS forum rather than in ABAP.