|PostgreSQL 8.1.23 Documentation|
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Table 9-26 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-25 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.
All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.
Table 9-25. Date/Time Operators
|+||date '2001-09-28' + integer '7'||date '2001-10-05'|
|+||date '2001-09-28' + interval '1 hour'||timestamp '2001-09-28 01:00:00'|
|+||date '2001-09-28' + time '03:00'||timestamp '2001-09-28 03:00:00'|
|+||interval '1 day' + interval '1 hour'||interval '1 day 01:00:00'|
|+||timestamp '2001-09-28 01:00' + interval '23 hours'||timestamp '2001-09-29 00:00:00'|
|+||time '01:00' + interval '3 hours'||time '04:00:00'|
|-||- interval '23 hours'||interval '-23:00:00'|
|-||date '2001-10-01' - date '2001-09-28'||integer '3'|
|-||date '2001-10-01' - integer '7'||date '2001-09-24'|
|-||date '2001-09-28' - interval '1 hour'||timestamp '2001-09-27 23:00:00'|
|-||time '05:00' - time '03:00'||interval '02:00:00'|
|-||time '05:00' - interval '2 hours'||time '03:00:00'|
|-||timestamp '2001-09-28 23:00' - interval '23 hours'||timestamp '2001-09-28 00:00:00'|
|-||interval '1 day' - interval '1 hour'||interval '1 day -01:00:00'|
|-||timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00'||interval '1 day 15:00:00'|
|*||900 * interval '1 second'||interval '00:15:00'|
|*||21 * interval '1 day'||interval '21 days'|
|*||double precision '3.5' * interval '1 hour'||interval '03:30:00'|
|/||interval '1 hour' / double precision '1.5'||interval '00:40:00'|
Table 9-26. Date/Time Functions
||interval||Subtract arguments, producing a "symbolic" result that uses years and months||age(timestamp '2001-04-10', timestamp '1957-06-13')||43 years 9 mons 27 days|
||age(timestamp '1957-06-13')||43 years 8 mons 3 days|
||date||Today's date; see Section 9.9.4|
||time with time zone||Time of day; see Section 9.9.4|
||timestamp with time zone||Date and time; see Section 9.9.4|
||double precision||Get subfield (equivalent to
||date_part('hour', timestamp '2001-02-16 20:38:40')||20|
||double precision||Get subfield (equivalent to
||date_part('month', interval '2 years 3 months')||3|
||timestamp||Truncate to specified precision; see also Section 9.9.2||date_trunc('hour', timestamp '2001-02-16 20:38:40')||2001-02-16 20:00:00|
||double precision||Get subfield; see Section 9.9.1||extract(hour from timestamp '2001-02-16 20:38:40')||20|
||double precision||Get subfield; see Section 9.9.1||extract(month from interval '2 years 3 months')||3|
||boolean||Test for finite time stamp (not equal to infinity)||isfinite(timestamp '2001-02-16 21:28:30')||true|
||boolean||Test for finite interval||isfinite(interval '4 hours')||true|
||interval||Adjust interval so 24-hour time periods are represented as days||justify_hours(interval '24 hours')||1 day|
||interval||Adjust interval so 30-day time periods are represented as months||justify_days(interval '30 days')||1 month|
||time||Time of day; see Section 9.9.4|
||timestamp||Date and time; see Section 9.9.4|
||timestamp with time zone||Current date and time (equivalent to
||text||Current date and time; see Section 9.9.4|
If you are using both
justify_days, it is best to use
justify_hours first so any additional days will
be included in the
In addition to these functions, the SQL OVERLAPS operator is supported:
(start1, end1) OVERLAPS (start2, end2) (start1, length1) OVERLAPS (start2, length2)
This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.
SELECT (DATE '2001-02-16', DATE '2001-12-21') OVERLAPS (DATE '2001-10-30', DATE '2002-10-30'); Result: true SELECT (DATE '2001-02-16', INTERVAL '100 days') OVERLAPS (DATE '2001-10-30', DATE '2002-10-30'); Result: false
When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.
EXTRACT(field FROM source)
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
returns values of type double precision.
The following are valid field names:
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. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.
PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.
The day (of the month) field (1 - 31)
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 16
The year field divided by 10
SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 200
The day of the week (0 - 6; Sunday is 0) (for timestamp values only)
SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 5
of the week numbering is different from that of the
The day of the year (1 - 365/366) (for timestamp values only)
SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 47
For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for interval values, 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';
The hour field (0 - 23)
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 20
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
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.
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
The minutes field (0 - 59)
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 38
For timestamp values, the number of the month within the year (1 - 12) ; for interval values 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
The quarter of the year (1 - 4) that the day is in (for timestamp values only)
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 1
The seconds field, including fractional parts (0 - 59)
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
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.
The hour component of the time zone offset
The minute component of the time zone offset
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 2006-01-01 is part of the 52nd week of year 2005.
SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 7
The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.
SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40'); Result: 2001
extract function is
primarily intended for computational processing. For formatting
date/time values for display, see Section 9.8.
date_part function is
modeled on the traditional Ingres equivalent to the SQL-standard function
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
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40'); Result: 16 SELECT date_part('hour', INTERVAL '4 hours 3 minutes'); Result: 4
conceptually similar to the
function for numbers.
source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically, to timestamp or interval respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).
Valid values for field are:
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40'); Result: 2001-02-16 20:00:00 SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40'); Result: 2001-01-01 00:00:00
The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-27 shows its variants.
Table 9-27. AT TIME ZONE Variants
|timestamp without time zone AT TIME ZONE zone||timestamp with time zone||Treat given time stamp without time zone as located in the specified time zone|
|timestamp with time zone AT TIME ZONE zone||timestamp without time zone||Convert given time stamp with time zone to the new time zone|
|time with time zone AT TIME ZONE zone||time with time zone||Convert given time with time zone to the new time zone|
In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, the available zone names are those shown in either Table B-6 or Table B-4.
Examples (supposing that the local time zone is PST8PDT):
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST'; Result: 2001-02-16 19:38:40-08 SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST'; Result: 2001-02-16 18:38:40
The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).
timezone(zone, timestamp) is equivalent to the
SQL-conforming construct timestamp AT TIME ZONE zone.
The following functions are available to obtain the current date and/or time:
CURRENT_DATE CURRENT_TIME CURRENT_TIMESTAMP CURRENT_TIME (precision) CURRENT_TIMESTAMP (precision) LOCALTIME LOCALTIMESTAMP LOCALTIME (precision) LOCALTIMESTAMP (precision)
CURRENT_TIMESTAMP deliver values with time
LOCALTIMESTAMP deliver values without time
LOCALTIMESTAMP can optionally be given a
precision parameter, which causes the result to be rounded to
that many fractional digits in the seconds field. Without a
precision parameter, the result is given to the full available
Note: Prior to PostgreSQL 7.2, the precision parameters were unimplemented, and the result was always given in integer seconds.
SELECT CURRENT_TIME; Result: 14:39:53.662522-05 SELECT CURRENT_DATE; Result: 2001-12-23 SELECT CURRENT_TIMESTAMP; Result: 2001-12-23 14:39:53.662522-05 SELECT CURRENT_TIMESTAMP(2); Result: 2001-12-23 14:39:53.66-05 SELECT LOCALTIMESTAMP; Result: 2001-12-23 14:39:53.662522
now() is the
It is important to know that
CURRENT_TIMESTAMP and related functions
return the start time of the current transaction; their values
do not change during the transaction. This is considered a
feature: the intent is to allow a single transaction to have a
consistent notion of the "current"
time, so that multiple modifications within the same
transaction bear the same time stamp.
Note: Other database systems may advance these values more frequently.
There is also the function
timeofday() which returns the wall-clock time
and advances during transactions. For historical reasons
timeofday() returns a text string rather than a timestamp value:
SELECT timeofday(); Result: Sat Feb 17 19:07:32.000126 2001 EST
All the date/time data types also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:
SELECT CURRENT_TIMESTAMP; SELECT now(); SELECT TIMESTAMP 'now'; -- incorrect for use with DEFAULT
Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.
60 if leap seconds are implemented by the operating system
The EXTRACT(EPOCH FROM INTERVAL ...) is a nice way to convert a timestamp to an int, i.e.
SELECT EXTRACT(EPOCH FROM INTERVAL '2 years 3 months') / (24*60*60)
will give the number of days in the interval (~821)
(Wouldn't it be great if (SELECT INTERVAL '2 years 3 months' / INTERAL '1 day') was supported?)
Here's a way to create an INTERVAL with an INT and a CHAR:
CAST(textcat(text(2), text(' seconds')) as interval)
You can use it thus:
SELECT current_timestamp + CAST(textcat(text(2), text(' seconds')) as interval)
If you use MS Access as GUI frontend by linked tables, Access will not allow update of a row if the row has timestamp field, like the one created by now(). The problem is the microsecond part of the timestamp as generally known. If you use timenow() instead, there'll be no microsecond part and Access will no more complain and allow updates. (The column type may does not need to change.)