//oracle中extract()函数从oracle 9i中引入,用于从一个date或者interval类型中截取到特定的部分
//语法如下：
EXTRACT (
{ YEAR | MONTH | DAY | HOUR | MINUTE | SECOND }
| { TIMEZONE_HOUR | TIMEZONE_MINUTE }
| { TIMEZONE_REGION | TIMEZONE_ABBR }
FROM { date_value | interval_value } )
//我们只可以从一个date类型中截取 year,month,day(date日期的格式为yyyy-mm-dd);
//我们只可以从一个 timestamp with time zone 的数据类型中截取TIMEZONE_HOUR和TIMEZONE_MINUTE;
select extract(year from date'2011-05-17') year from dual;
YEAR
2011
select extract(month from date'2011-05-17') month from dual;
MONTH
5
select extract(day from date'2011-05-17') day from dual;
DAY
17
//获取两个日期之间的具体时间间隔，extract函数是最好的选择
select extract(day from dt2-dt1) day
,extract(hour from dt2-dt1) hour
,extract(minute from dt2-dt1) minute
,extract(second from dt2-dt1) second
from (
select to_timestamp('2011-02-04 15:07:00','yyyy-mm-dd hh24:mi:ss') dt1
,to_timestamp('2011-05-17 19:08:46','yyyy-mm-dd hh24:mi:ss') dt2
from dual)
/
DAY HOUR MINUTE SECOND
102 4 1 46
select extract(year from systimestamp) year
,extract(month from systimestamp) month
,extract(day from systimestamp) day
,extract(minute from systimestamp) minute
,extract(second from systimestamp) second
,extract(timezone_hour from systimestamp) th
,extract(timezone_minute from systimestamp) tm
,extract(timezone_region from systimestamp) tr
,extract(timezone_abbr from systimestamp) ta
from dual
/
YEAR MONTH DAY MINUTE SECOND TH TM TR TA
2011 5 17 7 14.843 8 0 UNKNOWN UNK
//