oracle日期时间型timestamp相关学习

1、获取系统时间的语句(ssxff6获取小数点后面六位)  
select sysdate,systimestamp,to_char(systimestamp, 'yyyymmdd hh24:mi:ssxff6'), 
  to_char(systimestamp, 'yyyymmdd hh24:mi:ss.ff6') from dual;  

2、字符型转成timestamp 
select to_timestamp('2011-09-14 12:52:42.123456789', 'syyyy-mm-dd hh24:mi:ss.ff') from dual; 

3、timestamp转成date型  
select cast(to_timestamp('2011-09-14 12:52:42.123456789', 'syyyy-mm-dd hh24:mi:ss.ff') as date) timestamp_to_date from dual;  

4、date型转成timestamp  
select cast(sysdate as timestamp) date_to_timestamp from dual;  

5、两date的日期相减得出的是天数，而两timestamp的日期相减得出的是完整的年月日时分秒小数秒  
select sysdate-sysdate,systimestamp-systimestamp from dual; 

select extract(day from inter) * 24 * 60 * 60 +   
  extract(hour from inter) * 60 * 60 + extract(minute from inter) * 60 +   
    extract(second from inter) "seconds" from 
( 
  select to_timestamp('2011-09-14 12:34:23.281000000', 'yyyy-mm-dd hh24:mi:ss.ff') -   
    to_timestamp('2011-09-14 12:34:22.984000000', 'yyyy-mm-dd hh24:mi:ss.ff') inter from dual 
); 

select extract(second from to_timestamp('2011-09-14 12:34:23.281000000', 'yyyy-mm-dd hh24:mi:ss.ff'))- 
  extract(second from to_timestamp('2011-09-14 12:34:22.984000000', 'yyyy-mm-dd hh24:mi:ss.ff')) from dual;