add_months（date,n）

返回指定日期date加上n月后的日期

select add_months(to_date('20190321','yyyyMMdd'),22) from dual;--返回2021/1/21

last_day(date,n)

返回指定日期date减去n个月后的日期

select last_day(to_date('20190321','yyyyMMdd')) from dual;--返回指定日期月份的最后一天  2019/3/31

select last_day(to_date('20190321','yyyyMMdd')) from dual;--返回2019/3/31

months_between(d1,d2)

返回2个日期的差值

select months_between(to_date('20190321','yyyyMMdd'),to_date('20190421','yyyyMMdd')) from dual;--返回-1
select months_between(to_date('20190321','yyyyMMdd'),to_date('20190301','yyyyMMdd')) from dual;--返回0.645161290322581
select months_between(to_date('20190321','yyyyMMdd'),to_date('20190321','yyyyMMdd')) from dual;--返回0