Oracle 获取各类时间

1  SELECT to_char(TRUNC(SYSDATE - 4),'yyyy/mm/dd') current_day,
2              to_char(sysdate, 'yyyy/MM' ) current_month, 
3              to_char(sysdate,'yyyy/mm/dd') curr_day,
4              trunc(sysdate,'mm')+2 curr_day_start,
5              to_char(add_months(trunc(sysdate),-1),'yyyy/mm') last_month 
6  FROM dual;