Invoices Associated with the Given Purchase Order Number

SELECT aip.invoice_id invoice_id,
b.po_number po_number
FROM ap_invoices_all aip,
(SELECT invoice_id,po_number FROM ( SELECT ai.invoice_id,
AP_INVOICES_PKG.GET_PO_NUMBER( ai.invoice_id) po_number
FROM AP_INVOICES_ALL AI) A
WHERE a.po_number <>'UNMATCHED') b
where b.invoice_id=aip.invoice_id
and b.po_number= ‘Purchase Order Number’;