【www.bbyears.com--PostgreSQL】
一、获取系统时间函数
1.1 获取当前完整时间
代码如下select now();
david=# select now();
now
-------------------------------
2013-04-12 15:39:40.399711+08
(1 row)
david=# current_timestamp 同 now() 函数等效。
david=# select current_timestamp;
now
-------------------------------
2013-04-12 15:40:22.398709+08
(1 row)
david=# 1.2
获取当前日期
select current_date;
david=# select current_date;
date
------------
2013-04-12
(1 row)
david=# 1.3
获取当前时间
select current_time;
david=# select current_time;
timetz
--------------------
15:43:31.101726+08
(1 row)
david=#二、
时间的计算
代码如下
david=# select now();
now
-------------------------------
2013-04-12 15:47:13.244721+08
(1 row)
david=#2.1
两年后
代码如下
david=# select now() + interval "2 years";
?column?
-------------------------------
2015-04-12 15:49:03.168851+08
(1 row)
david=# select now() + interval "2 year";
?column?
-------------------------------
2015-04-12 15:49:12.378727+08
(1 row)
david=# select now() + interval "2 y";
?column?
------------------------------
2015-04-12 15:49:25.46986+08
(1 row)
david=# select now() + interval "2 Y";
?column?
-------------------------------
2015-04-12 15:49:28.410853+08
(1 row)
david=# select now() + interval "2Y";
?column?
-------------------------------
2015-04-12 15:49:31.122831+08
(1 row)
david=# 2.2
一个月后
代码如下
david=# select now() + interval "1 month";
?column?
------------------------------
2013-05-12 15:51:22.24373+08
(1 row)
david=# select now() + interval "one month";
ERROR: invalid input syntax for type interval: "one month"
LINE 1: select now() + interval "one month";
^
david=# 2.3
三周前
代码如下
david=# select now() - interval "3 week";
?column?
-------------------------------
2013-03-22 16:00:04.203735+08
(1 row)
david=# 2.4 十分钟后
david=# select now() + "10 min";
?column?
-------------------------------
2013-04-12 16:12:47.445744+08
(1 row)
david=#
说明：
interval 可以不写，其值可以是：
Abbreviation Meaning
Y Years
M Months (in the date part)
W Weeks
D Days
H Hours
M Minutes (in the time part)
S Seconds
2.5 计算两个时间差
使用 age(timestamp, timestamp)
代码如下
david=# select age(now(), timestamp "1989-02-05");
age
----------------------------------------
24 years 2 mons 7 days 17:05:49.119848
(1 row)
david=#
david=# select age(timestamp "2007-09-15");
age
------------------------
5 years 6 mons 27 days
(1 row)
david=#
三、时间字段的截取
在开发过程中，经常要取日期的年，月，日，小时等值，PostgreSQL 提供一个非常便利的EXTRACT函数。
EXTRACT(field FROM source)field 表示取的时间对象，source 表示取的日期来源，类型为 timestamp、time 或 interval。
3.1 取年份
代码如下
david=# select extract(year from now());
date_part
-----------
2013
(1 row)
david=# 3.2 取月份
david=# select extract(month from now());
date_part
-----------
4
(1 row)
david=#
david=# select extract(day from timestamp "2013-04-13");
date_part
-----------
13
(1 row)
david=#
david=# SELECT EXTRACT(DAY FROM INTERVAL "40 days 1 minute");
date_part
-----------
40
(1 row)
david=# 3.3 查看今天是一年中的第几天
david=# select extract(doy from now());
date_part
-----------
102
(1 row)
david=# 3.4
查看现在距1970-01-01 00:00:00 UTC 的秒数
david=# select extract(epoch from now());
date_part
------------------
1365755907.94474
(1 row)
david=# 3.5 把epoch 值转换回时间戳
david=# SELECT TIMESTAMP WITH TIME ZONE "epoch" + 1369755555 * INTERVAL "1 second";
?column?
------------------------
2013-05-28 23:39:15+08
(1 row)
david=#
--取day
skytf=> select extract(day from now());
date_part
-----------
3
(1 row)
skytf=> select extract(day from timestamp "2011-06-03");
date_part
-----------
3
(1 row)
skytf=> select timestamp "2011-06-03";
timestamp
---------------------
2011-06-03 00:00:00
(1 row)
--取小时
skytf=> select extract (hour from now());
date_part
-----------
14
(1 row)
--取分钟
skytf=> select extract (minute from now());
date_part
-----------
59
(1 row)
--取秒
skytf=> select extract (second from now());
date_part
-----------
46.039333
(1 row)
--取所在哪个星期
skytf=> select extract (week from now());
date_part
-----------
22
(1 row)
以上是基本的PG时间/日期函数使用，可满足一般的开发运维应用