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📜  PostgreSQL-日期/时间函数和运算符

📅  最后修改于: 2020-11-26 06:37:05             🧑  作者: Mango

我们在“数据类型”一章中讨论了日期/时间数据类型。现在，让我们看看日期/时间运算符和函数。

下表列出了基本算术运算运算符的行为-

Operator Example Result
+ 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’ (days)
– 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′

以下是所有重要的与日期和时间相关的可用功能的列表。

S. No. Function & Description
1 AGE()

Subtract arguments
2 CURRENT DATE/TIME()

Current date and time
3 DATE_PART()

Get subfield (equivalent to extract)
4 EXTRACT()

Get subfield
5 ISFINITE()

Test for finite date, time and interval (not +/-infinity)
6 JUSTIFY

Adjust interval

AGE(时间戳，时间戳)，AGE(时间戳)

S. No. Function & Description
1

AGE(timestamp, timestamp)

When invoked with the TIMESTAMP form of the second argument, AGE() subtract arguments, producing a “symbolic” result that uses years and months and is of type INTERVAL.
2

AGE(timestamp)

When invoked with only the TIMESTAMP as argument, AGE() subtracts from the current_date (at midnight).

函数AGE(timestamp，timestamp)的示例是-

testdb=# SELECT AGE(timestamp '2001-04-10', timestamp '1957-06-13');

上面给出的PostgreSQL语句将产生以下结果-

age
-------------------------
 43 years 9 mons 27 days

函数AGE(timestamp)的示例是-

testdb=# select age(timestamp '1957-06-13');

上面给出的PostgreSQL语句将产生以下结果-

age
--------------------------
 55 years 10 mons 22 days

当前日期/时间()

PostgreSQL提供了许多函数，这些函数返回与当前日期和时间有关的值。以下是一些功能-

S. No. Function & Description
1

CURRENT_DATE

Delivers current date.
2

CURRENT_TIME

Delivers values with time zone.
3

CURRENT_TIMESTAMP

Delivers values with time zone.
4

CURRENT_TIME(precision)

Optionally takes a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field.
5

CURRENT_TIMESTAMP(precision)

Optionally takes a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field.
6

LOCALTIME

Delivers values without time zone.
7

LOCALTIMESTAMP

Delivers values without time zone.
8

LOCALTIME(precision)

Optionally takes a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field.
9

LOCALTIMESTAMP(precision)

Optionally takes a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field.

使用上表中的功能的示例-

testdb=# SELECT CURRENT_TIME;
       timetz
--------------------
 08:01:34.656+05:30
(1 row)


testdb=# SELECT CURRENT_DATE;
    date
------------
 2013-05-05
(1 row)


testdb=# SELECT CURRENT_TIMESTAMP;
              now
-------------------------------
 2013-05-05 08:01:45.375+05:30
(1 row)


testdb=# SELECT CURRENT_TIMESTAMP(2);
         timestamptz
------------------------------
 2013-05-05 08:01:50.89+05:30
(1 row)


testdb=# SELECT LOCALTIMESTAMP;
       timestamp
------------------------
 2013-05-05 08:01:55.75
(1 row)

PostgreSQL还提供了返回当前语句开始时间的函数，以及该函数被调用时的实际当前时间。这些功能是-

S. No. Function & Description
1

transaction_timestamp()

It is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns.
2

statement_timestamp()

It returns the start time of the current statement.
3

clock_timestamp()

It returns the actual current time, and therefore its value changes even within a single SQL command.
4

timeofday()

It returns the actual current time, but as a formatted text string rather than a timestamp with time zone value.
5

now()

It is a traditional PostgreSQL equivalent to transaction_timestamp().

DATE_PART(文本，时间戳)，DATE_PART(文本，间隔)，DATE_TRUNC(文本，时间戳)

S. No. Function & Description
1

DATE_PART(‘field’, source)

These functions get the subfields. The field parameter needs to be a string value, not a name.

The valid field names are: century, day, decade, dow, doy, epoch, hour, isodow, isoyear, microseconds, millennium, milliseconds, minute, month, quarter, second, timezone, timezone_hour, timezone_minute, week, year.
2

DATE_TRUNC(‘field’, source)

This function is conceptually similar to the trunc function for numbers. source is a value expression of type timestamp or interval. field selects to which precision to truncate the input value. The return value is of type timestamp or interval.

The valid values for field are : microseconds, milliseconds, second, minute, hour, day, week, month, quarter, year, decade, century, millennium

以下是DATE_PART( ‘field’ ，source)函数的示例-

testdb=# SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
 date_part
-----------
        16
(1 row)


testdb=# SELECT date_part('hour', INTERVAL '4 hours 3 minutes');
 date_part
-----------
         4
(1 row)

以下是DATE_TRUNC( ‘field’ ，source)函数的示例-

testdb=# SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
     date_trunc
---------------------
 2001-02-16 20:00:00
(1 row)


testdb=# SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
     date_trunc
---------------------
 2001-01-01 00:00:00
(1 row)

EXTRACT(时间戳字段)，EXTRACT(间隔字段)

EXTRACT(field FROM source)函数从日期/时间值中检索子字段，例如年或小时。源必须是timestamp，time或interval类型的值表达式。该字段是标识符或字符串，用于选择要从源值提取的字段。 EXTRACT函数返回双精度类型的值。

以下是有效的字段名称(类似于DATE_PART函数的字段名称)：世纪，日期，十年，陶氏，doy，纪元，小时，等距，等年，微秒，千年，毫秒，分钟，月份，四分之一秒，时区，timezone_hour ，timezone_minute，星期，年份。

以下是EXTRACT( ‘field’ ，source)函数的示例-

testdb=# SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13');
 date_part
-----------
        20
(1 row)


testdb=# SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
 date_part
-----------
        16
(1 row)

ISFINITE(日期)，ISFINITE(时间戳)，ISFINITE(间隔)

S. No. Function & Description
1

ISFINITE(date)

Tests for finite date.
2

ISFINITE(timestamp)

Tests for finite time stamp.
3

ISFINITE(interval)

Tests for finite interval.

以下是ISFINITE()函数的示例-

testdb=# SELECT isfinite(date '2001-02-16');
 isfinite
----------
 t
(1 row)


testdb=# SELECT isfinite(timestamp '2001-02-16 21:28:30');
 isfinite
----------
 t
(1 row)


testdb=# SELECT isfinite(interval '4 hours');
 isfinite
----------
 t
(1 row)

JUSTIFY_DAYS(间隔)，JUSTIFY_HOURS(间隔)，JUSTIFY_INTERVAL(间隔)

S. No. Function & Description
1

JUSTIFY_DAYS(interval)

Adjusts interval so 30-day time periods are represented as months. Return the interval type
2

JUSTIFY_HOURS(interval)

Adjusts interval so 24-hour time periods are represented as days. Return the interval type
3

JUSTIFY_INTERVAL(interval)

Adjusts interval using JUSTIFY_DAYS and JUSTIFY_HOURS, with additional sign adjustments. Return the interval type

以下是ISFINITE()函数的示例-

testdb=# SELECT justify_days(interval '35 days');
 justify_days
--------------
 1 mon 5 days
(1 row)


testdb=# SELECT justify_hours(interval '27 hours');
 justify_hours
----------------
 1 day 03:00:00
(1 row)


testdb=# SELECT justify_interval(interval '1 mon -1 hour');
 justify_interval
------------------
 29 days 23:00:00
(1 row)