枚举上月的日期

有学生提这个需求，这个太好实现了，肯定是没好好听课

SELECT last_m + (LEVEL - 1) AS d
  FROM (SELECT add_months(trunc(SYSDATE, 'mm'), -1) last_m,
               trunc(SYSDATE, 'mm') m
          FROM dual)
CONNECT BY LEVEL <= m - last_m