trunc

日期处理：

        trunc（x[,fmt]）用于对x截断，在不指定fmt参数时默认截断为当天的0点(也就是默认fmt参数为'DD')，fmt是可选的字符串参数，它指明要截断的单元，返回的结果为data类型。例如，MM表示截断到当月的第一天。这个函数对NLS_CALENDAR会话参数不敏感，它按照公历的规则运行。

SQL> select systimestamp from dual;

SYSTIMESTAMP
------------------------------------
21-MAR-19 10.56.11.105051 AM +08:00

SQL> select  trunc(systimestamp) from dual;

TRUNC(SYSTIMESTAMP)
-------------------
2019-03-21 00:00:00

SQL> select  trunc(systimestamp,'MM') from dual;

TRUNC(SYSTIMESTAMP,
-------------------
2019-03-01 00:00:00

SQL> select  trunc(systimestamp,'D') from dual;

TRUNC(SYSTIMESTAMP,
-------------------
2019-03-17 00:00:00

SQL> select  trunc(systimestamp,'DD') from dual;

TRUNC(SYSTIMESTAMP,
-------------------
2019-03-21 00:00:00

SQL> select  trunc(systimestamp,'DDD') from dual;

TRUNC(SYSTIMESTAMP,
-------------------
2019-03-21 00:00:00

SQL> select  trunc(systimestamp,'J') from dual;

TRUNC(SYSTIMESTAMP,
-------------------
2019-03-21 00:00:00

SQL> select  trunc(systimestamp,'HH24') from dual;

TRUNC(SYSTIMESTAMP,
-------------------
2019-03-21 10:00:00

SQL> select  trunc(systimestamp,'MI') from dual;

TRUNC(SYSTIMESTAMP,
-------------------
2019-03-21 10:57:00