我们已经讨论过的章节中数据类型的日期/时间（DATE/TIME）数据类型。现在让我们来看日期/时间运算符和函数。
下表列出了基本算术操作符的行为：
| 运算符 | 实例 | 结果 |
|---|---|---|
| + | 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′ |
以下是所有重要的日期和时间相关的可用功能列表中。
| 函数 | 描述 |
|---|---|
| AGE() | Subtract arguments |
| CURRENT DATE/TIME() | Current date and time |
| DATE_PART() | Get subfield (equivalent to extract) |
| EXTRACT() | Get subfield |
| ISFINITE() | Test for finite date,time and interval (not +/-infinity) |
| JUSTIFY | Adjust interval |
AGE(timestamp, timestamp),
AGE(timestamp)
| 函数 | 描述 |
|---|---|
| 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. |
| AGE(timestamp) | When invoked with only the TIMESTAMP as argument, AGE() subtracts from the current_date (at midnight). |
例如时代功能 AGE(timestamp, timestamp)是：
testdb=# SELECT AGE(timestamp '2001-04-10', timestamp '1957-06-13');
以上PostgreSQL的表会产生以下结果：
age
-------------------------
43 years 9 mons 27 days
功能AGE（timestamp ）的例子是：
testdb=# select age(timestamp '1957-06-13');
Above PostgreSQL statement will produce following result:
age
--------------------------
55 years 10 mons 22 days
CURRENT DATE/TIME()
PostgreSQL提供了返回值的函数的当前日期和时间相关。以下是一些函数：
| 函数 | 描述 |
|---|---|
| CURRENT_DATE | Delivers current date. |
| CURRENT_TIME | Deliver values with time zone. |
| CURRENT_TIMESTAMP | Deliver values with time zone. |
| CURRENT_TIME(precision) | Optionally takes a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. |
| CURRENT_TIMESTAMP(precision) | Optionally takes a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. |
| LOCALTIME | Deliver values without time zone. |
| LOCALTIMESTAMP | Deliver values without time zone. |
| LOCALTIME(precision) | Optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. |
| LOCALTIMESTAMP(precision) | Optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. |
例子使用的功能表所示：
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还提供了当前语句的开始时间，以及当前的实际时间的瞬间，该函数被调用的函数返回。这些函数包括：
| 函数 | 描述 |
|---|---|
| transaction_timestamp() | 它相当于CURRENT_TIMESTAMP，但被命名为清楚地反映它返回什么。 |
| statement_timestamp() | 它返回当前语句的开始时间。 |
| clock_timestamp() | It returns the actual current time, and therefore its value changes even within a single SQL command. |
| timeofday() | It returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. |
| now() | It is a traditional PostgreSQL equivalent to transaction_timestamp(). |
DATE_PART(text, timestamp),
DATE_PART(text, interval),
DATE_TRUNC(text, timestamp)
| Function | Description |
|---|---|
| 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. |
| DATE_TRUNC(‘field’, source) | This function is conceptually similar to the trunc function for numbers. sourceis 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 orinterval. The valid values for field are : microseconds, milliseconds, second, minute, hour, day, week, month, quarter, year, decade, century, millennium |
Following are examples for DATE_PART(‘field’, source) functions:
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)
Following are examples for DATE_TRUNC(‘field’, source) functions:
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(field from timestamp),
EXTRACT(field from interval)
The EXTRACT(field FROM source) function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. 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 (similar to DATE_PART function field names):century, day, decade, dow, doy, epoch, hour, isodow, isoyear, microseconds, millennium, milliseconds, minute, month, quarter, second, timezone, timezone_hour, timezone_minute, week, year.
以下是EXTRACT(‘field’, source) 函数的例子:
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(date),
ISFINITE(timestamp),
ISFINITE(interval)
| 函数 | 描述 |
|---|---|
| ISFINITE(date) | Tests for finite date. |
| ISFINITE(timestamp) | Tests for finite time stamp. |
| ISFINITE(interval) | Tests for finite interval. |
以下是为ISFINITE()函数的例子：
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(interval),
JUSTIFY_HOURS(interval),
JUSTIFY_INTERVAL(interval)
| 函数 | 描述 |
|---|---|
| JUSTIFY_DAYS(interval) | Adjusts interval so 30-day time periods are represented as months. Return the interval type |
| JUSTIFY_HOURS(interval) | Adjusts interval so 24-hour time periods are represented as days. Return the interval type |
| JUSTIFY_INTERVAL(interval) | Adjusts interval using JUSTIFY_DAYS and JUSTIFY_HOURS, with additional sign adjustments. Return the interval type |
以下是为ISFINITE（）函数的例子：
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)
