Oracle 中与时间有关的计算

# sysdate+1/24        - 加1小时
# sysdate+1/24/60     - 加1分钟
# sysdate+1/24/60/60  - 加1秒钟
# 
SQL> alter session set nls_date_format='yyyy-MM-dd hh24:mi:ss';

Session altered.

SQL> select sysdate, sysdate+1/24, sysdate+1/24/60, sysdate+1/24/60/60 from dual;

SYSDATE             SYSDATE+1/24        SYSDATE+1/24/60     SYSDATE+1/24/60/60
------------------- ------------------- ------------------- -------------------
2013-09-18 09:40:43 2013-09-18 10:40:43 2013-09-18 09:41:43 2013-09-18 09:40:44

SQL> -- 获取本周第一天
SQL> select trunc(sysdate, 'day') from dual;

TRUNC(SYSDATE,'DAY'                                                             
-------------------                                                             
2013-10-13 00:00:00                                                             

SQL> -- 获取本周最后一天
SQL> select trunc(sysdate, 'day') + 7 - 1/24/60/60 from dual;

TRUNC(SYSDATE,'DAY'                                                             
-------------------                                                             
2013-10-19 23:59:59                                                             

SQL> -- 获取下周第 n 天
SQL> select trunc(next_day(sysdate, 1)), trunc(next_day(sysdate, 7)) from dual;

TRUNC(NEXT_DAY(SYSD TRUNC(NEXT_DAY(SYSD                                         
------------------- -------------------                                         
2013-10-20 00:00:00 2013-10-26 00:00:00                                         

SQL> --
SQL> -- 获取上月第一天
SQL> select trunc(add_months(sysdate, -1), 'month') from dual;

TRUNC(ADD_MONTHS(SY                                                             
-------------------                                                             
2013-09-01 00:00:00                                                             

SQL> select to_date(to_char(add_months(sysdate, -1), 'yyyy-MM'), 'yyyy-MM') from dual;

TO_DATE(TO_CHAR(ADD                                                             
-------------------                                                             
2013-09-01 00:00:00                                                             

SQL> -- 获取本月第一天
SQL> select trunc(sysdate, 'month') from dual;

TRUNC(SYSDATE,'MONT                                                             
-------------------                                                             
2013-10-01 00:00:00                                                             

SQL> select to_date(to_char(sysdate, 'yyyy-MM'), 'yyyy-MM') from dual;

TO_DATE(TO_CHAR(SYS                                                             
-------------------                                                             
2013-10-01 00:00:00                                                             

SQL> -- 获取本月最后一天
SQL> select trunc(last_day(sysdate)) + 1 - 1/24/60/60 from dual;

TRUNC(LAST_DAY(SYSD                                                             
-------------------                                                             
2013-10-31 23:59:59                                                             

SQL> select add_months(trunc(sysdate, 'month'), 1) - 1/24/60/60 from dual;

ADD_MONTHS(TRUNC(SY                                                             
-------------------                                                             
2013-10-31 23:59:59                                                             

SQL> --
SQL> -- 获取本季度第一天
SQL> select trunc(sysdate, 'Q') from dual;

TRUNC(SYSDATE,'Q')                                                              
-------------------                                                             
2013-10-01 00:00:00                                                             

SQL> -- 获取本季度最后一天
SQL> select add_months(trunc(sysdate, 'Q'), 3) - 1/24/60/60 from dual;

ADD_MONTHS(TRUNC(SY                                                             
-------------------                                                             
2013-12-31 23:59:59                                                             

SQL> --
SQL> -- 获取去年第一天(因为闰年平年天数不一样，所以不能使用加减 366或365天 来计算年份)
SQL> select trunc(trunc(sysdate, 'year') - 1, 'year') from dual;

TRUNC(TRUNC(SYSDATE                                                             
-------------------                                                             
2012-01-01 00:00:00                                                             

SQL> -- 获取今年第一天
SQL> select trunc(sysdate, 'year') from dual;

TRUNC(SYSDATE,'YEAR                                                             
-------------------                                                             
2013-01-01 00:00:00                                                             

SQL> select to_date(to_char(sysdate, 'yyyy')||'-01-01', 'yyyy-MM-dd') from dual;

TO_DATE(TO_CHAR(SYS                                                             
-------------------                                                             
2013-01-01 00:00:00                                                             

SQL> -- 获取今年最后一天
SQL> select trunc(add_months(sysdate, 12), 'year') - 1/24/60/60 from dual;

TRUNC(ADD_MONTHS(SY                                                             
-------------------                                                             
2013-12-31 23:59:59                                                             

SQL> select to_date(to_char(sysdate, 'yyyy')||'-12-31 23:59:59', 'yyyy-MM-dd hh24:mi:ss') from dual;

TO_DATE(TO_CHAR(SYS                                                             
-------------------                                                             
2013-12-31 23:59:59                                                             

SQL> 
SQL> -- 计算相差的天数
SQL> select round(to_number(to_date('2013-10-19 23:59:59', 'yyyy-MM-dd hh24:mi:ss') - to_date('2013-10-18 00:00:00', 'yyyy-MM-dd hh24:mi:ss'))) value from dual;

     VALUE                                                                      
----------                                                                      
         2                                                                      

SQL> -- 计算相差的小时数
SQL> select round(to_number(to_date('2013-10-19 12:00:00', 'yyyy-MM-dd hh24:mi:ss') - to_date('2013-10-19 11:00:00', 'yyyy-MM-dd hh24:mi:ss')) * 24) value from dual;

     VALUE                                                                      
----------                                                                      
         1                                                                      

SQL> -- 计算相差的分钟数
SQL> select round(to_number(to_date('2013-10-19 12:00:00', 'yyyy-MM-dd hh24:mi:ss') - to_date('2013-10-19 11:30:00', 'yyyy-MM-dd hh24:mi:ss')) * 24 * 60) value from dual;

     VALUE                                                                      
----------                                                                      
        30                                                                      

SQL> 

# 相关链接

ROUND(date) http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions154.htm#SQLRF00699 

TRUNC(date) http://docs.oracle.com/cd/E11882_01/server.112/e41084/functions220.htm#SQLRF06151