SQL语句——04、日期类型函数

如果你对当前系统的日期格式看这不舒服，可以修改当前会话的显示格式

idle> alter session set NLS_DATE_FORMAT=‘YYYY-MM-DD HH24:MI:SS‘;

Session altered.

idle> select sysdate from dual;

SYSDATE
-------------------
2010-12-17 08:52:31

这是OS系统时间，数据库本身没时间 只有SCN号

日期可以直接参与运算

idle> select sysdate-10 from dual;

SYSDATE-10
-------------------
2010-12-07 08:53:16

两个日期型数据相减会得到相差的天数

idle> select to_date(‘2010-12-30‘)-sysdate from dual;

TO_DATE(‘2010-12-30‘)-SYSDATE
-----------------------------
     12.6290856

idle>

可以将日期型的数据和一个小时数相加减 这个数要除以24

idle> select sysdate from dual;

SYSDATE
-------------------
2010-12-17 08:55:56

idle> select sysdate + 5/24 from dual;

SYSDATE+5/24
-------------------
2010-12-17 13:55:56

idle>

MONTHS_BEWTEEN(日期1,日期2)

如果日期1大于日期2返回正数,日期1小于日期2返回负数

idle> select months_between(‘2010-10-10‘,‘2010-12-10‘) from dual;

MONTHS_BETWEEN(‘2010-10-10‘,‘2010-12-10‘)
-----------------------------------------
           -2

idle> select months_between(‘2010-12-10‘,‘2010-10-10‘) from dual;

MONTHS_BETWEEN(‘2010-12-10‘,‘2010-10-10‘)
-----------------------------------------
     2

idle>

ADD_MONTHS(日期,n)

把n个月加到日期上

idle> select add_months(‘2010-10-10‘,3) from dual;

ADD_MONTHS(‘2010-10
-------------------
2011-01-10 00:00:00

NEXT_DAY(日期,星期)

下一个星期几的日期? 如果是中文系统将MONDAY改成"星期一"

idle> select next_day(sysdate,‘MONDAY‘) from dual;

NEXT_DAY(SYSDATE,‘M
-------------------
2010-12-20 09:02:06

idle>
idle> select next_day(sysdate,‘fri‘) from dual;

NEXT_DAY(SYSDATE,‘F
-------------------
2010-12-24 09:02:44

idle>

LAST_DAY(日期)

返回该日期的所在月的最后一天

idle> select last_day(sysdate) from dual;

LAST_DAY(SYSDATE)
-------------------
2010-12-31 09:03:26

idle>

idle> alter session set nls_date_format=‘YYYY-MM-DD‘;

Session altered.
idle> select ename,hiredate,last_day(hiredate),next_day(hiredate,‘SUN‘),months_between(sysdate,hiredate) "MON",ADD_MONTHS(hiredate,3) from emp
where ename=‘SCOTT‘;

ENAME   HIREDATE   LAST_DAY(H NEXT_DAY(H   MON ADD_MONTHS
---------- ---------- ---------- ---------- ---------- ----------
SCOTT   1987-04-19 1987-04-30 1987-04-26 283.947709 1987-07-19

idle>

ROUND(date,‘[day|month|year]‘) 和 trunc(date,‘[day|month|year]‘)

应用于日期型数据，数字的进位和截取是以小数点为中心,而日期的进位和截取是以年月日时分秒为中心

idle> select round(‘2010-10-10‘,‘MONTH‘) from dual;
select round(‘2010-10-10‘,‘MONTH‘) from dual
             *
ERROR at line 1:
ORA-01722: invalid number
因为round不能处理字符型数据,所以报错,我们要将字符型数据转换为日期型

idle> select round(to_date(‘2010-10-10‘),‘MONTH‘) from dual;

ROUND(TO_DATE(
-------------------
2010-10-01 00:00:00

这是按月为单位进行四舍五入 因为10号是16号以前 所以得到10.1日
月为单位的四舍五入的分割线
idle>  select round(to_date(‘2010-10-15‘),‘MONTH‘),trunc(to_date(‘2010-10-15‘)) from dual;

ROUND(TO_DATE(2010 TRUNC(TO_DATE(2010
------------------- -------------------
2010-10-01 00:00:00 2010-10-15 00:00:00

idle>  select round(to_date(‘2010-10-16‘),‘MONTH‘),trunc(to_date(‘2010-10-16‘)) from dual;

ROUND(TO_DATE(2010 TRUNC(TO_DATE(2010
------------------- -------------------
2010-11-01 00:00:00 2010-10-16 00:00:00

idle>

年为单位的四舍五入分割线
idle> select round(to_date(‘2010-06-30‘),‘year‘),trunc(to_date(‘2010-06-30‘),‘year‘) from dual;

ROUND(TO_DATE(2010 TRUNC(TO_DATE(2010
------------------- -------------------
2010-01-01 00:00:00 2010-01-01 00:00:00

idle>  select round(to_date(‘2010-07-01‘),‘year‘),trunc(to_date(‘2010-07-01‘),‘year‘) from dual;

ROUND(TO_DATE(2010 TRUNC(TO_DATE(2010
------------------- -------------------
2011-01-01 00:00:00 2010-01-01 00:00:00

idle>

RR与 YY的区别

SQL> select to_date(‘18-may-91‘,‘DD-MON-RR‘) FROM DUAL;

TO_DATE(‘18-MAY-91‘
-------------------
1991-05-18 00:00:00

SQL> select to_date(‘18-may-91‘,‘DD-MON-YY‘) FROM DUAL;

TO_DATE(‘18-MAY-91‘
-------------------
2091-05-18 00:00:00

SQL> 

原文地址：https://www.cnblogs.com/marxist/p/11708150.html