oracle date-time

select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss')

 

select * from tablename where time>=  to_date('2011-05-02','yyyy-mm-dd')    

 

select * from tablename where to_char(time,'yyyy-mm-dd')>='2011-05-02'    and to_char(time,'yyyy-mm-dd')<='2011-05-30'