取出当前时间上个月的最后一天

2012年的面试题，可惜没写出来啊

SQL> select sysdate from dual;

SYSDATE
-------------------
2016-11-09 09:42:44

SQL> select trunc(sysdate) from dual;

TRUNC(SYSDATE)
-------------------
2016-11-09 00:00:00

SQL> select add_months(sysdate,-1) from dual;

ADD_MONTHS(SYSDATE,
-------------------
2016-10-09 09:43:57

SQL> select last_day(add_months(sysdate,-1)) from dual;

LAST_DAY(ADD_MONTHS
-------------------
2016-10-31 09:44:22

SQL> select trunc(last_day(add_months(sysdate,-1))) from dual;

TRUNC(LAST_DAY(ADD_
-------------------
2016-10-31 00:00:00

SQL> select trunc(last_day(add_months(sysdate,-1)))-1/86400 from dual;

TRUNC(LAST_DAY(ADD_
-------------------
2016-10-30 23:59:59

SQL> select sysdate,trunc(last_day(add_months(sysdate,-1)))-1/86400 from dual;

SYSDATE TRUNC(LAST_DAY(ADD_
------------------- -------------------
2016-11-09 09:46:20 2016-10-30 23:59:59