关于Oracle日期格式（DD-MM-YY）转化为YYYY-mm-DD

一、DD-MM-RR

SELECT TO_CHAR(TO_DATE('12-01-49', 'DD-MM-RR'), 'YYYYMMDD') "Year",TO_CHAR(TO_DATE('12-01-98', 'DD-MM-RR'), 'YYYYMMDD') "Year" FROM DUAL;

二、DD-MM-YY

SELECT TO_CHAR(TO_DATE('12-01-49', 'DD-MM-YY'), 'YYYYMMDD') "Year",TO_CHAR(TO_DATE('12-01-98', 'DD-MM-YY'), 'YYYYMMDD') "Year" FROM DUAL;

三、to_char 和 to_date

   to_char: 将时间格式转化为字符格式

   to_date：将字符格式转化为时间格式

四、 trunc

select  sysdate, trunc(sysdate) from dual

五、extract

select  extract(Year from sysdate) from dual