-- http://www.gurubee.net/lecture/1903 
WITH T(emp_no, emp_name, dept, ent_year, manager_no) AS (
SELECT '18001', '김땡땡', '회계', '2018', '17001' FROM dual UNION ALL 
SELECT '18002', '이땡땡', '인사', '2018', '17001' FROM dual UNION ALL
SELECT '17001', '박땡땡', '회계', '2017', '15001' FROM dual UNION ALL
SELECT '15001', '최땡땡', '회계', '2018', '02003' FROM dual UNION ALL
SELECT '02003', '한땡땡', '회계', '2018', '' FROM dual 
)

SELECT LPAD(' ', 4*(LEVEL-1)) || emp_name emp_name, emp_no,
CONNECT_BY_ROOT  emp_no "Root emp_no", level
  FROM T
  WHERE dept = '회계'
 START WITH manager_no IS NULL 
CONNECT BY PRIOR emp_no=manager_no;

결과가 좀 이상한데요?
"2018년에 입사한 사원 중에 부서가 회계팀인 사원"은 총 3명인데? 결과는 왜 1건이죠?
혹시 최땡땡과 한땡땡의 입사년도 '2018' 은 오타인가요?

WITH t(emp_no, emp_name, dept, ent_year, manager_no) AS
(
SELECT '18001', '김땡땡', '회계', '2018', '17001' FROM dual UNION ALL
SELECT '18002', '이땡땡', '인사', '2018', '17001' FROM dual UNION ALL
SELECT '17001', '박땡땡', '회계', '2017', '15001' FROM dual UNION ALL
SELECT '15001', '최땡땡', '회계', '2015', '02003' FROM dual UNION ALL -- 오타 수정
SELECT '02003', '한땡땡', '회계', '2002', ''      FROM dual           -- 오타 수정
)
SELECT CONNECT_BY_ROOT(emp_no  ) emp_no
     , CONNECT_BY_ROOT(emp_name) emp_name
     , emp_no   manager_no
     , emp_name manager_name
  FROM t
 WHERE CONNECT_BY_ISLEAF = 1
 START WITH ent_year = '2018' AND dept = '회계'
 CONNECT BY PRIOR manager_no = emp_no
;