Oracle日期转字符串

SELECT sysdate,to_date((TO_CHAR(SYSDATE,'YYYY-MM-DD')||' 14:30:00'),'YYYY-MM-DD HH24:MI:SS' ) FROM DUAL;

SELECT sysdate,to_date((TO_CHAR(SYSDATE,'YYYY-MM-DD')||' 10:30:00'),'YYYY-MM-DD HH12:MI:SS' ) FROM DUAL;