예시처럼 종료일자가 이미 들어 있는 상황이라면?
종료일자 그대로 뽑으면 되겠는데요? 종료일 없으면 시작일로 대체
NVL(종료일자, 시작일자)

WITH t AS
(
SELECT 1 eno, '재임용' app, '20200401' sdt, 1 seq, '20210331' edt FROM dual
UNION ALL SELECT 1, '퇴직'    , '20190331', 1, ''         FROM dual
UNION ALL SELECT 1, '계약변경', '20180401', 3, '20190331' FROM dual
UNION ALL SELECT 1, '재임용'  , '20180401', 2, '20190331' FROM dual
UNION ALL SELECT 1, '퇴직'    , '20180401', 1, ''         FROM dual
UNION ALL SELECT 1, '계약변경', '20170401', 2, '20180331' FROM dual
UNION ALL SELECT 1, '신규임용', '20170401', 1, '20180331' FROM dual
)
SELECT eno, app, sdt, seq, edt
     , SUBSTR(
       LAST_VALUE(DECODE(app, '퇴직', sdt)) IGNORE NULLS OVER
       (PARTITION BY eno ORDER BY sdt DESC, seq DESC)
       , 1, 6) rym
  FROM t
 WHERE eno = 1
 ORDER BY sdt DESC
;