months_between()

格式：MONTHS_BETWEEN(DATE1,DATE2)

MONTHS_BETWEEN函数返回两个日期之间的月份数。
SQL> select months_between(to_date('20090228', 'yyyymmdd'), to_date('20080228', 'yyyymmdd')) as months from dual;

    MONTHS
----------
        12

SQL> select months_between(to_date('20090228', 'yyyymmdd'), to_date('20080229', 'yyyymmdd')) as months from dual;

    MONTHS
----------
        12

SQL> select months_between(to_date('20080229', 'yyyymmdd'), to_date('20070228', 'yyyymmdd')) as months from dual;

    MONTHS
----------
        12

SQL> select months_between(to_date('20100331', 'yyyymmdd'), to_date('20100228', 'yyyymmdd')) as months from dual;

    MONTHS
----------
         1

SQL> select months_between(to_date('20100228', 'yyyymmdd'), to_date('20100128', 'yyyymmdd')) as months from dual;

    MONTHS
----------
         1

SQL> select months_between(to_date('20100327', 'yyyymmdd'), to_date('20100228', 'yyyymmdd')) as months from dual;

    MONTHS
----------
.967741935

SQL> select months_between(to_date('20100330', 'yyyymmdd'), to_date('20100228', 'yyyymmdd')) as months from dual;

    MONTHS
----------
1.06451613

SQL> select months_between(to_date('20100228', 'yyyymmdd'), to_date('20100130', 'yyyymmdd')) as months from dual;

    MONTHS
----------
.935483871

SQL> select months_between(to_date('20100228', 'yyyymmdd'), to_date('20100131', 'yyyymmdd')) as months from dual;

    MONTHS
----------
         1

SQL> select months_between(to_date('20100228', 'yyyymmdd'), to_date('20100127', 'yyyymmdd')) as months from dual;

    MONTHS
----------
1.03225806

30/31=0.96774193548387096774193548387097
33/31=1.0645161290322580645161290322581
29/31=0.93548387096774193548387096774194
32/31=1.032258064516129032258064516129
在ORACLE中计算月份的差值，都是月底或是日期相同，都会按整月计算。
如果计算结果不是整数，ORACLE会把31作为分母来计算