Table 4-14 shows the available functions for date/time value processing. The basic arithmetic operators (+, *, etc.) are also available. For formatting functions, refer to Section 4.6. You should be familiar with the background information on date/time data types (see Section 3.4).
Table 4-14. Date/Time Functions
|age(timestamp)||interval||Subtract from today||age(timestamp '1957-06-13')||43 years 8 mons 3 days|
|age(timestamp, timestamp)||interval||Subtract arguments||age('2001-04-10', timestamp '1957-06-13')||43 years 9 mons 27 days|
|current_date||date||Today's date; see below|
|current_time||time||Time of day; see below|
|current_timestamp||timestamp||date and time; see also below|
|date_part(text, timestamp)||double precision||Get subfield (equivalent to extract); see also below||date_part('hour', timestamp '2001-02-16 20:38:40')||20|
|date_part(text, interval)||double precision||Get subfield (equivalent to extract); see also below||date_part('month', interval '2 years 3 months')||3|
|date_trunc(text, timestamp)||timestamp||Truncate to specified precision; see also below||date_trunc('hour', timestamp '2001-02-16 20:38:40')||2001-02-16 20:00:00+00|
|extract(field from timestamp)||double precision||Get subfield; see also below||extract(hour from timestamp '2001-02-16 20:38:40')||20|
|extract(field from interval)||double precision||Get subfield; see also below||extract(month from interval '2 years 3 months')||3|
|isfinite(timestamp)||boolean||Test for finite time stamp (neither invalid nor infinity)||isfinite(timestamp '2001-02-16 21:28:30')||true|
|isfinite(interval)||boolean||Test for finite interval||isfinite(interval '4 hours')||true|
|now()||timestamp||Current date and time (equivalent to current_timestamp); see also below|
|timeofday()||text||High-precision date and time; see also below||timeofday()||Wed Feb 21 17:01:13.000126 2001 EST|
|timestamp(date)||timestamp||Date to timestamp||timestamp(date '2000-12-25')||2000-12-25 00:00:00|
|timestamp(date, time)||timestamp||Date and time to a timestamp||timestamp(date '1998-02-24',time '23:07')||1998-02-24 23:07:00|
EXTRACT (field FROM source)
The extract function retrieves sub-fields from date/time values, such as year or hour. source is a value expression that evaluates to type timestamp or interval. (Expressions of type date or time 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 values:
The year field divided by 100
Note that the result for the century field is simply the year field divided by 100, and not the conventional definition which puts most years in the 1900's in the twentieth century.
The day (of the month) field (1 - 31)
The year field divided by 10
The day of the week (0 - 6; Sunday is 0) (for timestamp values only)
The day of the year (1 - 365/366) (for timestamp values only)
For date and timestamp values, the number of seconds since 1970-01-01 00:00:00 (Result may be negative.); for interval values, the total number of seconds in the interval
The hour field (0 - 23)
The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.
The year field divided by 1000
Note that the result for the millennium field is simply the year field divided by 1000, and not the conventional definition which puts years in the 1900's in the second millennium.
The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.
The minutes field (0 - 59)
For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)
The quarter of the year (1 - 4) that the day is in (for timestamp values only)
The seconds field, including fractional parts (0 - 59)
From a timestamp value, calculate 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 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year.
The year field
The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 4.6.
The date_part function is modeled on the traditional Ingres equivalent to the SQL-function extract:
date_part('field', source)Note that here the field value needs to be a string. The valid field values for date_part are the same as for extract.
The function date_trunc is conceptually similar to the trunc function for numbers.
date_trunc('field', source)source is a value expression of type timestamp (values of type date and time are cast automatically). field selects to which precision to truncate the time stamp value. The return value is of type timestamp with all fields that are less than the selected one set to zero (or one, for day and month).
Valid values for field are:
The following functions are available to obtain the current date and/or time:
CURRENT_TIME CURRENT_DATE CURRENT_TIMESTAMPNote that because of the requirements of the SQL standard, these functions must not be called with trailing parentheses.
SELECT CURRENT_TIME; 19:07:32 SELECT CURRENT_DATE; 2001-02-17 SELECT CURRENT_TIMESTAMP; 2001-02-17 19:07:32-05
The function now() is the traditional Postgres equivalent to CURRENT_TIMESTAMP.
There is also timeofday(), which returns current time to higher precision than the CURRENT_TIMESTAMP family does:
timeofday() uses the operating system call gettimeofday(2), which may have resolution as good as microseconds (depending on your platform); the other functions rely on time(2) which is restricted to one-second resolution. For historical reasons, timeofday() returns its result as a text string rather than a timestamp value.
It is quite important to realize that CURRENT_TIMESTAMP and related functions all return the time as of the start of the current transaction; their values do not increment while a transaction is running. But timeofday() returns the actual current time.
All the date/time datatypes 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';
Note: You do not want to use the third form when specifying a DEFAULT value 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
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