Unassigned to Buyer PRs

Dear Team,

How we can modify this query , to find out below PRs unassigned to buyer. below data only for unassigned PRs to buyer.

SELECT segment1 pr_number,

       description pr_description,

       approved_date pr_approved_date,

       (SELECT SUM (NVL (quantity * unit_price, 0))

FROM po_requisition_lines_all

WHERE requisition_header_id =

prh.requisition_header_id) pr_total_amount,

       (select full_name from per_people_x where person_id = preparer_id) requester,

       attribute5 department_name

  FROM PO_REQUISITION_HEADERS_ALL prh order by prh.requisition_header_id desc

Please check and advise.

Thanks