Oracle 21c

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CREATE TABLE Table1 (PO, RequestedDate) AS
SELECT 14888, DATE '2018-01-12' FROM DUAL UNION ALL
SELECT 14733, DATE '2018-02-12' FROM DUAL UNION ALL
SELECT 14555, DATE '2018-05-12' FROM DUAL;
3 rows affected

CREATE TABLE Table2 (PO, ReceivedDate) AS
SELECT 14888, DATE '2018-01-11' FROM DUAL UNION ALL
SELECT 14888, DATE '2018-01-14' FROM DUAL UNION ALL
SELECT 14733, DATE '2018-02-11' FROM DUAL UNION ALL
SELECT 14733, DATE '2018-02-12' FROM DUAL UNION ALL
SELECT 14555, DATE '2018-07-23' FROM DUAL UNION ALL
SELECT 14555, DATE '2018-09-23' FROM DUAL;
6 rows affected

with t as (
  select
    po,
    /*Prepend numeric constant for stable sort order*/
    '0_REQ' as src,
    RequestedDate as dt
  from table1
  
  union all
  
  select
    po,
    '1_RCV' as src,
    ReceivedDate as dt
  from table2
)
select /*+gather_plan_statistics*/ *
from t
match_recognize(
  partition by po
  /*Order by dates, then by event type*/
  order by dt asc, src asc
  measures
    /*Requested date is taken from a request event*/
    req.dt as RequestedDate,
    /*Received date is taken from a receive event*/
    rcv.dt as ReceivedDate
  pattern (req rcv)
  define
    req as src = '0_REQ',
    rcv as src = '1_RCV' 
)


PO
REQUESTEDDATE
RECEIVEDDATE


14555
12-MAY-18
23-JUL-18

14733
12-FEB-18
12-FEB-18

14888
12-JAN-18
14-JAN-18

PO	REQUESTEDDATE	RECEIVEDDATE
14555	12-MAY-18	23-JUL-18
14733	12-FEB-18	12-FEB-18
14888	12-JAN-18	14-JAN-18