日期 function

SELECT 
SYSDATE,
ADD_MONTHS(SYSDATE,2),
ADD_MONTHS(SYSDATE,-2),
LAST_DAY(SYSDATE),
MONTHS_BETWEEN(SYSDATE,DATE'2015-12-22'),
MONTHS_BETWEEN(SYSDATE,DATE'2020-12-22'),
NEXT_DAY(SYSDATE,'SUNDAY'),
NEXT_DAY(SYSDATE,'SUN'), 
TRUNC(SYSDATE),
TRUNC(SYSDATE,'hh'),
TRUNC(SYSDATE,'month'),
TRUNC(SYSDATE,'YEAR'),
round(sysdate,'year')
FROM DUAL;

SYSDATE   ADD_MONTHS(SYSDATE,2) ADD_MONTHS(SYSDATE,-2) LAST_DAY(SYSDATE) MONTHS_BETWEEN(SYSDATE,DATE'2015-12-22') MONTHS_BETWEEN(SYSDATE,DATE'2020-12-22') NEXT_DAY(SYSDATE,'SUNDAY') NEXT_DAY(SYSDATE,'SUN') TRUNC(SYSDATE) TRUNC(SYSDATE,'HH') TRUNC(SYSDATE,'MONTH') TRUNC(SYSDATE,'YEAR') ROUND(SYSDATE,'YEAR')
--------- --------------------- ---------------------- ----------------- ---------------------------------------- ---------------------------------------- -------------------------- ----------------------- -------------- ------------------- ---------------------- --------------------- ---------------------
03-SEP-18 03-NOV-18             03-JUL-18              30-SEP-18                                       32.4008778                              -27.5991222 09-SEP-18                  09-SEP-18               03-SEP-18      03-SEP-18           01-SEP-18              01-JAN-18             01-JAN-19