Form Personalization: writing sql query in message type

Hi Team

I have a requirement , where i need to show receipt no of a PO.

whenever user navigate to Receiving>Receipts and he enters PO number. System then checks for receipt no ,then there only i need to show message .

"Receipt no x, y,z has been created in system".

query:

'select distinct receipt_num

FROM   po_headers_all pha,

              rcv_shipment_lines  rsl,

              rcv_shipment_headers rsh

WHERE  1=1

AND     pha.po_header_id=rsl.po_header_id

AND     rsl.shipment_header_id=rsh.shipment_header_id

AND     pha.segment1=:OVERFLOW_REGION.ORDER_NUM'

Later i wil use listagg  but before this , can anyone suggest me.