WITH t AS
(
SELECT 1 pk, '20180501' sdt, '20200531' edt FROM dual
)
SELECT pk
     , GREATEST(sdt, (SUBSTR(sdt, 1, 4) + lv - 1) || '0101') sdt
     , LEAST   (edt, (SUBSTR(sdt, 1, 4) + lv - 1) || '1231') edt
  FROM t
     , (SELECT LEVEL lv FROM dual CONNECT BY LEVEL <= 20)
 WHERE lv <= SUBSTR(edt, 1, 4) - SUBSTR(sdt, 1, 4) + 1
 ORDER BY pk, sdt
;