You have a purchase order and you have a requisition, but wait how do you know how this purchase order is linked with requisition. Here is the query thats answers this. Use this query find linked Purchase order and Requisition.

This could be really a helpful one.

SELECT prh.segment1 req_number
      ,prl.line_num req_line_num
      ,prl.item_description req_item_description
      ,prl.unit_price req_unit_price
      ,prl.quantity req_quantity
      ,ph.segment1 po_number
  FROM po_requisition_headers_all prh
      ,po_requisition_lines_all   prl
      ,po_req_distributions_all   prd
      ,po_distributions_all       pd
      ,po_line_locations_all      pll
      ,po_lines_all           pl
      ,po_headers_all             ph
 WHERE prh.requisition_header_id = prl.requisition_header_id
   and prh.org_id = prl.org_id
   and prl.requisition_line_id = prd.requisition_line_id
   and prl.org_id = prd.org_id
   and prd.distribution_id = pd.req_distribution_id(+)
   and prd.org_id = pd.org_id(+)
   and pd.line_location_id = pll.line_location_id(+)
   and pd.org_id = pll.org_id(+)
   and pll.po_line_id = pl.po_line_id(+)
   and pll.org_id = pl.org_id(+)
   and pl.po_header_id = ph.po_header_id(+)
   and pl.org_id = ph.org_id(+)

Based on a request from one of the reader here is the query which he was looking for.

He needed query that can list all the responsibilities attached to a user.

select fu.user_name, fr.responsibility_name, furg.START_DATE, furg.END_DATE
from fnd_user_resp_groups_direct furg, fnd_user fu, fnd_responsibility_tl fr
where fu.user_user_name = :user_name
and furg.user_id = fu.user_id
and furg.responsibility_id = fr.responsibility_id
and fr.language = userenv(‘LANG’)