Hope this helps

SELECT distinct ain.invoice_num "Invoice Number"
,pha.SEGMENT1 "PO Number"
-- NEED PO NUMBER ALSO
FROM   po_vendor_sites_all pvs,
      ap_invoices_all ain
      ,pa_expenditure_items_all pei
      ,pa_expenditures_all pe
      ,ap_invoice_distributions_all aida
      ,po_distributions_all pda
      ,po_headers_all pha     
 WHERE pei.expenditure_id = pe.expenditure_id
   AND pei.transaction_source = 'AP INVOICE'
   AND ain.invoice_num = pe.orig_user_exp_txn_reference
   and ain.VENDOR_site_id = pvs.VENDOR_SITE_ID
   and ain.INVOICE_ID = aida.INVOICE_ID
   and aida.PO_DISTRIBUTION_ID = pda.PO_DISTRIBUTION_ID
   and pda.PO_HEADER_ID = pha.PO_HEADER_ID;

Thnaks,

DK