-- EXTRACT函数：返回具体的年、月、日、时、分、秒等具体时间。
SELECT EXTRACT (YEAR FROM --SYSDATE
       SYSTIMESTAMP
       ) YEAR FROM DUAL;
       
SELECT EXTRACT (MONTH FROM --SYSDATE
       SYSTIMESTAMP
       ) YEAR FROM DUAL;
       
SELECT EXTRACT (DAY FROM --SYSDATE
       SYSTIMESTAMP
       ) YEAR FROM DUAL;
       
SELECT EXTRACT (HOUR FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT (MINUTE FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT (SECOND FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT (DAY FROM SYSTIMESTAMP) YEAR FROM DUAL;

SELECT EXTRACT(YEAR FROM ORDERDATE) AS O

--EG:
SELECT EXTRACT(YEAR FROM SYSDATE) AS YEAR_,EXTRACT(MONTH FROM SYSDATE) AS MONTH_,EXTRACT(DAY FROM SYSDATE) AS DAY_
       FROM DUAL