Query to get Requisition Number from PO Number

TO track Requisition NUMBER FROM PO NUMBER
===========================================

SELECT segment1
  FROM po_requisition_headers_all
 WHERE requisition_header_id IN (SELECT requisition_header_id
                                   FROM po_requisition_lines_all
                                  WHERE requisition_line_id IN (SELECT requisition_line_id
                                                                  FROM po_req_distributions_all
                                                                 WHERE distribution_id IN
                                                                 (SELECT req_distribution_id
                                                                    FROM po_distributions_all
                                                                   WHERE po_header_id IN
                                                                                 (SELECT po_header_id
                                                                                    FROM po_headers_all
                                                                                   WHERE segment1 =<<Give PO Number>>)
)
)
);