The query below extract all Test Case ID under a Specific Folder, and all its descendants, from TestPlan section.

Prerequisit: Start Folder Node ID

Suppose the Node ID is 5. This is the Query:

SELECT TS_TEST_ID FROM TEST
WHERE 

TS_SUBJECT IN

        (SELECT AL_ITEM_ID FROM ALL_LISTS 

         WHERE

         AL_ABSOLUTE_PATH LIKE (SELECT AL_ABSOLUTE_PATH+'%'

                                             FROM ALL_LISTS WHERE AL_ITEM_ID = 5))