PostgreSQL的时间/日期函数使用
PostgreSQL的常用时间函数使用整理如下:
一、获取系统时间函数
1.1 获取当前完整时间
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
david=# select now(); now ------------------------------- 2013-04-12 15:47:13.244721+08 (1 row) david=#
2.1 两年后
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
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
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)
david=# select age(now(), timestamp '1989-02-05'); age ---------------------------------------- years 2 mons 7 days 17:05:49.119848 (1 row) david=#
david=# select age(timestamp '2007-09-15'); age ------------------------ 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
david=# select extract(year from now()); date_part ----------- (1 row) david=#
3.2 取月份
david=# select extract(month from now()); date_part ----------- (1 row) david=#
david=# select extract(day from timestamp '2013-04-13'); date_part ----------- (1 row) david=#
david=# SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute'); date_part ----------- (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 ----------- (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=#
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。
详细用法请参考:
PostgreSQL官方说明:http://www.postgresql.org/docs/9.2/static/functions-datetime.html
转载 http://www.cnblogs.com/mchina/archive/2013/04/15/3010418.html