trunc与round

select trunc(sysdate) from dual; --2017/11/7 今天的日期为2017-11-07
select trunc(sysdate, ‘mm‘) from dual; --2017/11/1 返回当月第一天.
select trunc(sysdate,‘yy‘) from dual; --2017/1/1 返回当年第一天
select trunc(sysdate,‘dd‘) from dual; --2017/11/7 返回当前年月日
select trunc(sysdate,‘yyyy‘) from dual; --2017/1/1 返回当年第一天
select trunc(sysdate,‘d‘) from dual; --2017/11/5 (星期天)返回当前星期的第一天
select trunc(sysdate, ‘hh‘) from dual; --2017/11/7 17:00:00 当前时间为17:34 
select trunc(sysdate, ‘mi‘) from dual; --2017/11/7 17:34:00 TRUNC()函数没有秒的精确

select trunc(123.458) from dual; --123
select trunc(123.458,0) from dual; --123
select trunc(123.458,1) from dual; --123.4
select trunc(123.458,-1) from dual; --120
select trunc(123.458,-4) from dual; --0
select trunc(123.458,4) from dual; --123.458
select trunc(123) from dual; --123
select trunc(123,1) from dual; --123
select trunc(123,-1) from dual; --120

select round(123.456) from dual; --123
select round(123.456,0) from dual; --123
select round(123.456,1) from dual; --123.5
select round(123.456,2) from dual; --123.46
select round(123.456,-1) from dual; --120
select round(123.456,-2) from dual; --100
select round(123,1) from dual; --123
select round(123,-1) from dual; --120