参考地址:https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions201.htm

TRUNC (date)Description of the illustration trunc_date.gif

"ROUND and TRUNC Date Functions" for the permitted format models to use in fmt.

Examples

The following example truncates a date:

SELECT TRUNC(TO_DATE('27-OCT-92','DD-MON-YY'), 'YEAR')
  "New Year" FROM DUAL;
 
New Year
---------
01-JAN-92 

SELECT TRUNC(TO_DATE('27-OCT-92','DD-MON-YY'), 'YEAR')
  "New Year" FROM DUAL;
 
New Year
---------
01-JAN-92