select t.* from tb t where not exists(select 1 from tb where trunc(add_months(sysdate,-1),'mm')=trunc(dt_col,'mm') and dt_col>t.dt_col)

------解决方案--------------------


--取上个月的值,用函数add_months()
select last_month from tab where add_months(sysdate,-1)=last_month

--只取一条,用row_number()或是min,max的函数
select last_month
from
(
 select last_month,row_number()over(order by last_month) rn from tab where   add_months(sysdate,-1)=last_month
)
where rn=1



------解决方案--------------------

select * from  
(select * from tb1 a where add_months(sysdate,-1)=a.date order by a.id)
where rownum<2

------解决方案--------------------

SQL code
select id,name,tel,createtime
from
(
select id,name,tel,createtime,row_number()over(partition by createtime) rn from tab1
where trunc(createtime,'mm')=trunc(add_months(sysdate,-1),'mm')
)
where rn=1

------解决方案--------------------

实测成功：
SQL code

CREATE TABLE tab1
(
    ID NUMBER(4),
    NAME VARCHAR2(100),
    tel VARCHAR2(20),
    createtime DATE
);
DELETE tab1;
INSERT INTO tab1 VALUES(1, 'John', '111111', to_date('2011-10-13 20:00:00', 'YYYY-MM-DD HH24:MI:SS'));
INSERT INTO tab1 VALUES(2, 'Peter', '222222', to_date('2011-10-13 20:10:00', 'YYYY-MM-DD HH24:MI:SS'));
INSERT INTO tab1 VALUES(3, 'Tom', '333333', to_date('2011-10-14 20:15:00', 'YYYY-MM-DD HH24:MI:SS'));
INSERT INTO tab1 VALUES(4, 'Adward', '444444', to_date('2011-11-20 17:15:00', 'YYYY-MM-DD HH24:MI:SS'));

SELECT * FROM tab1;

SELECT * FROM tab1
WHERE createtime = 
    (SELECT MIN(createtime) 
     FROM tab1 
     WHERE to_char(createtime, 'YYYY-MM') = to_char(add_months(SYSDATE, -1), 'YYYY-MM'));