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

Oracle 查询三个月之内