以上內容轉自：http://blog.csdn.net/suncrafted/article/details/4273528

select trunc(sysdate,‘yy‘) from dual ;-- 本年第一天
select trunc(sysdate,‘mm‘) from dual; --本月第一天
select trunc(sysdate,‘day‘)+1 from dual;-- 本周第一天

--一年有多少個星期
with x0 as 
(select to_date(‘2018-01-01‘,‘yyyy-mm-dd‘) as 年初 from dual ),
x1 as 
(select 年初,add_months(年初,12) as 下年年初 from x0),
x2 as 
(select 年初,下年年初,下年年初-年初 as 天數  from x1),
x3 as 
(select 年初+level-1 as 日期 from x2  connect by level <=天數 ),
x4 as 
(select 日期,to_char(日期,‘DAY‘) AS 星期 FROM X3)

SELECT 星期,count(*) from x4 group by 星期

（012）每日SQL學習：TO_CHAR(DATE,FORMAT)