--截取后得到的仍为date数据类型

select trunc(sysdate) from dual;--2017-03-13 00:00:00
select trunc(sysdate+1) from dual;--2017-03-14 00:00:00 加一天

select trunc(sysdate,'yyyy') from dual;--2017-01-01 00:00:00
select trunc(sysdate,'MM') from dual;--2017-03-01 00:00:00
select trunc(sysdate,'DD') from dual;--2017-03-13 00:00:00

select trunc(sysdate,'HH') from dual;--2017-03-13 11:00:00
select trunc(sysdate,'MI') from dual;--2017-03-13 11:34:00
select trunc(sysdate,'ss') from dual;--精度说明符错错误
select sysdate+1 from dual;--2017-03-13 11:36:10
select sysdate+1 from dual;--2017-03-14 11:36:10 加一天