WITH t AS
(
SELECT '201601' ym, '수월한검사' gb FROM dual
UNION ALL SELECT '201602', '수월한검사' FROM dual
UNION ALL SELECT '201603', '수월한검사' FROM dual
UNION ALL SELECT '201604', '수월한검사' FROM dual
UNION ALL SELECT '201605', '보통검사'   FROM dual
UNION ALL SELECT '201606', '수월한검사' FROM dual
UNION ALL SELECT '201607', '수월한검사' FROM dual
)
SELECT SUM(FLOOR(COUNT(*)/3)) cnt
  FROM (SELECT ym FROM t WHERE gb = '수월한검사' ORDER BY ym)
 GROUP BY ADD_MONTHS(TO_DATE(ym, 'yyyymm'), -ROWNUM)
;