Oracle 18c

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CREATE TABLE dates(alldates date);
INSERT INTO dates  (alldates) VALUES (DATE '2017-05-01');
1 rows affected

INSERT INTO dates  (alldates) VALUES (DATE '2018-03-01');
1 rows affected

WITH t1(test)  AS (
  SELECT MIN(alldates)
  FROM dates 
UNION ALL 
  SELECT ADD_MONTHS(test,1)
  FROM t1 
  WHERE ADD_MONTHS(test,1) <= (SELECT MAX(alldates) FROM dates)
)
SELECT * FROM t1


TEST


01-MAY-17

01-JUN-17

01-JUL-17

01-AUG-17

01-SEP-17

01-OCT-17

01-NOV-17

01-DEC-17

01-JAN-18

01-FEB-18

01-MAR-18


WITH t1(min_date, max_date)  AS (
  SELECT MIN(alldates),
         MAX(alldates)  
  FROM   dates 
UNION ALL 
  SELECT ADD_MONTHS(min_date,1),
         max_date
  FROM   t1 
  WHERE  ADD_MONTHS(min_date,1) <= max_date
)
SELECT min_date
FROM   t1


MIN_DATE


01-MAY-17

01-JUN-17

01-JUL-17

01-AUG-17

01-SEP-17

01-OCT-17

01-NOV-17

01-DEC-17

01-JAN-18

01-FEB-18

01-MAR-18