months_between(date1,date2) 老师说返回日期想减，怎么写
SQL> select '31-1月-19'-'23-6月-17' from dual;
select '31-1月-19'-'23-6月-17' from dual
ORA-01722: 无效数字
SQL> select months_between('31-5月-18'-'21-6月-17') from dual;
select months_between('31-5月-18'-'21-6月-17') from dual
ORA-00909: 参数个数无效
