自带函数：add_months(sysdate,-3)
select add_months(sysdate,-3) from dual
Time Start: 2015/1/12 15:00:38
SYSDATE ADD_MONTHS(SYSDATE,-3)
---------- ----------------------
2015-01-12 2014-10-12
1 row selected.
Time End: 2015/1/12 15:00:38
Elapsed Time for Script Execution: 73 msecs