round(date)函数，将时间进行四舍五入，测试如下：
SQL> select sysdate from dual;
SYSDATE
-----------------
20090327 10:48:45
SQL> select round(sysdate,'year') from dual;
ROUND(SYSDATE,'YE
-----------------
20090101 00:00:00
SQL> select round(sysdate,'mon') from dual;
ROUND(SYSDATE,'MO
-----------------
20090401 00:00:00
SQL> select round(sysdate,'dd') from dual;
ROUND(SYSDATE,'DD
-----------------
20090327 00:00:00
SQL> select round(sysdate,'hh24') from dual;
ROUND(SYSDATE,'HH
-----------------
20090327 11:00:00
SQL> select round(sysdate,'mi') from dual;
ROUND(SYSDATE,'MI
-----------------
20090327 10:50:00
SQL> select sysdate from dual;
SYSDATE
-----------------
20090327 10:50:07
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