一。类型转换
postgres的类型转换：通常::用来做类型转换，timestamp到date用的比较多
select now()::date
select now()::varchar
示例1：日期的varchar计算成date
select '2012-11-15 16:15:56.377000+08'::timestamp::date
select '2012-11-15 16:15:56.377000+08'::date
结果: 2012-11-15
二。时间的类型转换与相对时间
//注意java的timestamp将来在sql中体现的varchar的形式‘2012-11-15 16:15:56.377000+08’，这样的串可以计算时间差。
假如表中的一条记录的publishdate是 '2012-11-15 16:15:56.377000+08'，想确认该记录是不是过去24小时之内publish的记录，可以使用如下的判断：
select extract(epoch from now() - '2012-11-15 16:15:56.377000+08')< 24*3600
select now() - '2012-11-15 16:15:56.377000+08' < '24 hours'
select now() - '2012-11-15 16:15:56.377000+08' < '1 days' or select now() - '2012-11-16 16:15:56.377000+08' < '1 day'
select now()::date-'2012-11-15 16:15:56.377000+08'::date < 1
注：相对时间表示时间范围，通常用于统计，定时任务 。除了相对时间，‘today’使用的也比较多。比如取当天的记录使用:publishdate::date = 'today'
三。时间函数Extract用于提取绝对时间的年，月，日.....; 相对时间的秒值。
EXTRACT(field FROM source)
The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:
century The century
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13'); Result: 20 SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 21
The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 to 1. PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100. day The day (of the month) field (1--31)
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 16
decade The year field divided by 10
SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 200
dow The day of the week (0--6; Sunday is 0) (for timestampvalues only)
SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 5
Note that extract's day of the week numbering is different from that of the to_char function. doy The day of the year (1--365/366) (for timestampvalues only)
SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 47
epoch For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for intervalvalues, the total number of seconds in the interval
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-08'); Result: 982384720 SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours'); Result: 442800
Here is how you can convert an epoch value back to a time stamp:
SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720 * INTERVAL '1 second';
hour The hour field (0--23)
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 20
microseconds The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.
SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5'); Result: 28500000
millennium The millennium
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 3
Years in the 1900s are in the second millennium. The third millennium starts January 1, 2001. PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000. milliseconds The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.
SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5'); Result: 28500
minute The minutes field (0--59)
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 38
month For timestamp values, the number of the month within the year (1--12) ; for intervalvalues the number of months, modulo 12 (0--11)
SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 2 SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months'); Result: 3 SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months'); Result: 1
quarter The quarter of the year (1--4) that the day is in (for timestampvalues only)
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 1
second The seconds field, including fractional parts (0 - 59(3))
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 40 SELECT EXTRACT(SECOND FROM TIME '17:12:28.5'); Result: 28.5
timezone The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. timezone_hour The hour component of the time zone offset timezone_minute The minute component of the time zone offset week The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year. (for timestamp values only) Because of this, it is possible for early January dates to be part of the 52nd or 53rd week of the previous year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01is part of the 52nd week of year 2005.
SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 7
year The year field. Keep in mind there is no 0 AD, so subtracting BC years from ADyears should be done with care.
SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 2001
The extract function is primarily intended for computational processing. For formatting date/time values for display, see section 7.8 Data Type Formatting Functions.
The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:
date_part('field', source)Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40'); Result: 16 SELECT date_part('hour', INTERVAL '4 hours 3 minutes'); Result: 4What Doesn't Kill Me Makes Me Stronger
