PostgreSQL tutorial of vi: functions and operators in detail (2)
- 2020-05-06 11:54:15
- OfStack
Pattern matching:
Three approaches to pattern matching are provided in PostgreSQL: the SQL LIKE operator, the more recent SIMILAR TO operator, and the POSIX-style regular expression.
1. LIKE:
string LIKE pattern [ ESCAPE escape-character ]
string NOT LIKE pattern [ ESCAPE escape-character ]
each pattern defines a collection of strings. The LIKE expression returns true if the string is included in the string collection represented by pattern. As we might expect, if LIKE returns true, then NOT LIKE expression returns false and vice versa. The underscore (_) in pattern matches any single character, while a percent (%) matches any zero or more characters, such as
'abc' LIKE 'abc' true
'abc' LIKE 'a%' true
'abc' LIKE '_b_' true
'abc' LIKE 'c' false
matches the underscore or percent sign of the text rather than any other character, and the corresponding character in pattern must lead to an escape character. The default escape character is a backslash, but you can specify one with the ESCAPE clause. To match the escape character itself, write two escape characters. We can also effectively turn off the escape mechanism by writing ESCAPE ", where we cannot turn off the special meanings of underscores and percentages.
The keyword ILIKE can be used to replace LIKE, making the match case-insensitive for the current locale. This feature is not an SQL standard, but an extension of PostgreSQL. The operator ~~ is equivalent to LIKE, while ~~* corresponds to ILIKE. There are! ~ ~ and! The ~~* operators represent NOT LIKE and NOT ILIKE, respectively. All of these operators are PostgreSQL specific.
2. SIMILAR TO regular expression:
SIMILAR TO returns true or false depending on whether the pattern matches a given string.
string SIMILAR TO pattern [ESCAPE escape-character]
string NOT SIMILAR TO pattern [ESCAPE escape-character]
It is very similar to LIKE in that it supports LIKE's wildcards ('_' and '%') and maintains its original meaning. In addition, SIMILAR TO also supports some of its own unique metacharacters, such as
1). | id selection (one of two candidates).
2).
3). + means to repeat the previous item once or more times.
4). You can use parentheses () to combine items into a logical item.
5). A square bracket expression [...] Declare a list of characters, just like the POSIX regular expression.
see the following example:
'abc' SIMILAR TO 'abc' true
'abc' SIMILAR TO 'a' false
'abc' SIMILAR TO '%(b|d)%' true
'abc' SIMILAR TO '(b|c)%' false
substring, substring(string from pattern for escape-character), with three arguments, provides a function to extract a substring from a string that matches the SQL regular expression pattern. As with SIMILAR TO, the declared pattern must match the entire data string, otherwise the function fails and returns NULL. To identify the part of the pattern that should be returned on success, the pattern must appear with two escape characters followed by double quotes ("). A string matching the pattern between the two tokens is returned, such as
MyTest=# SELECT substring('foobar' from '% "o_b#"%' FOR '#'); -- here # is an escape, and the pattern in double quotes is the return part.
substring
-----------
oob
(1 row)
MyTest=# SELECT substring('foobar' from '#"o_b#"%' FOR '#'); --foobar Cannot exactly match the following pattern, so returns NULL .
substring
-----------
(1 row)
Data type formatting function:
The PostgreSQL formatting function provides an effective set of tools for converting various data types (date/time, integer, floating point, numeric) to a formatted string and, in turn, from a formatted string to a specified data type. These functions are listed below, and they all follow a common calling convention: the first parameter is the value to be formatted, and the second is the template that defines the output or output format.
| 函数 | 返回类型 | 描述 | 例子 |
| to_char(timestamp, text) | text | 把时间戳转换成字串 | to_char(current_timestamp, 'HH12:MI:SS') |
| to_char(interval, text) | text | 把时间间隔转为字串 | to_char(interval '15h 2m 12s', 'HH24:MI:SS') |
| to_char(int, text) | text | 把整数转换成字串 | to_char(125, '999') |
| to_char(double precision, text) | text | 把实数/双精度数转换成字串 | to_char(125.8::real, '999D9') |
| to_char(numeric, text) | text | 把numeric转换成字串 | to_char(-125.8, '999D99S') |
| to_date(text, text) | date | 把字串转换成日期 | to_date('05 Dec 2000', 'DD Mon YYYY') |
| to_timestamp(text, text) | timestamp | 把字串转换成时间戳 | to_timestamp('05 Dec 2000', 'DD Mon YYYY') |
| to_timestamp(double) | timestamp | 把UNIX纪元转换成时间戳 | to_timestamp(200120400) |
| to_number(text, text) | numeric | 把字串转换成numeric | to_number('12,454.8-', '99G999D9S') |
1. Pattern for date/time formatting:
| 模式 | 描述 |
| HH | 一天的小时数(01-12) |
| HH12 | 一天的小时数(01-12) |
| HH24 | 一天的小时数(00-23) |
| MI | 分钟(00-59) |
| SS | 秒(00-59) |
| MS | 毫秒(000-999) |
| US | 微秒(000000-999999) |
| AM | 正午标识(大写) |
| Y,YYY | 带逗号的年(4和更多位) |
| YYYY | 年(4和更多位) |
| YYY | 年的后三位 |
| YY | 年的后两位 |
| Y | 年的最后一位 |
| MONTH | 全长大写月份名(空白填充为9字符) |
| Month | 全长混合大小写月份名(空白填充为9字符) |
| month | 全长小写月份名(空白填充为9字符) |
| MON | 大写缩写月份名(3字符) |
| Mon | 缩写混合大小写月份名(3字符) |
| mon | 小写缩写月份名(3字符) |
| MM | 月份号(01-12) |
| DAY | 全长大写日期名(空白填充为9字符) |
| Day | 全长混合大小写日期名(空白填充为9字符) |
| day | 全长小写日期名(空白填充为9字符) |
| DY | 缩写大写日期名(3字符) |
| Dy | 缩写混合大小写日期名(3字符) |
| dy | 缩写小写日期名(3字符) |
| DDD | 一年里的日子(001-366) |
| DD | 一个月里的日子(01-31) |
| D | 一周里的日子(1-7；周日是1) |
| W | 一个月里的周数(1-5)(第一周从该月第一天开始) |
| WW | 一年里的周数(1-53)(第一周从该年的第一天开始) |
2. Template pattern for numeric formatting:
| 模式 | 描述 |
| 9 | 带有指定数值位数的值 |
| 0 | 带前导零的值 |
| .(句点) | 小数点 |
| ,(逗号) | 分组(千)分隔符 |
| PR | 尖括号内负值 |
| S | 带符号的数值 |
| L | 货币符号 |
| D | 小数点 |
| G | 分组分隔符 |
| MI | 在指明的位置的负号(如果数字 < 0) |
| PL | 在指明的位置的正号(如果数字 > 0) |
| SG | 在指明的位置的正/负号 |
8, time/date function and operator:
1. Below is a list of supported time/date operators in PostgreSQL:
| 操作符 | 例子 | 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' |
| + | date '2001-09-28' + time '03:00' | timestamp '2001-09-28 03:00' |
| + | interval '1 day' + interval '1 hour' | interval '1 day 01:00' |
| + | timestamp '2001-09-28 01:00' + interval '23 hours' | timestamp '2001-09-29 00:00' |
| + | time '01:00' + interval '3 hours' | time '04:00' |
| - | - interval '23 hours' | interval '-23:00' |
| - | 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' |
| - | time '05:00' - time '03:00' | interval '02:00' |
| - | time '05:00' - interval '2 hours' | time '03:00' |
| - | timestamp '2001-09-28 23:00' - interval '23 hours' | timestamp '2001-09-28 00:00' |
| - | interval '1 day' - interval '1 hour' | interval '23:00' |
| - | timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00' | interval '1 day 15:00' |
| * | interval '1 hour' * double precision '3.5' | interval '03:30' |
| / | interval '1 hour' / double precision '1.5' | interval '00:40' |
2. Date/time function:
| 函数 | 返回类型 | 描述 | 例子 | 结果 |
| age(timestamp, timestamp) | interval | 减去参数，生成一个使用年、月的"符号化"的结果 | age('2001-04-10', timestamp '1957-06-13') | 43 years 9 mons 27 days |
| age(timestamp) | interval | 从current_date减去得到的数值 | age(timestamp '1957-06-13') | 43 years 8 mons 3 days |
| current_date | date | 今天的日期 | ||
| current_time | time | 现在的时间 | ||
| current_timestamp | timestamp | 日期和时间 | ||
| date_part(text, timestamp) | double | 获取子域(等效于extract) | date_part('hour', timestamp '2001-02-16 20:38:40') | 20 |
| date_part(text, interval) | double | 获取子域(等效于extract) | date_part('month', interval '2 years 3 months') | 3 |
| date_trunc(text, timestamp) | timestamp | 截断成指定的精度 | date_trunc('hour', timestamp '2001-02-16 20:38:40') | 2001-02-16 20:00:00+00 |
| extract(field from timestamp) | double | 获取子域 | extract(hour from timestamp '2001-02-16 20:38:40') | 20 |
| extract(field from interval) | double | 获取子域 | extract(month from interval '2 years 3 months') | 3 |
| localtime | time | 今日的时间 | ||
| localtimestamp | timestamp | 日期和时间 | ||
| now() | timestamp | 当前的日期和时间(等效于 current_timestamp) | ||
| timeofday() | text | 当前日期和时间 |
3. EXTRACT, date_part function support field:
| 域 | 描述 | 例子 | 结果 |
| CENTURY | 世纪 | EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13'); | 20 |
| DAY | (月分)里的日期域(1-31) | EXTRACT(DAY from TIMESTAMP '2001-02-16 20:38:40'); | 16 |
| DECADE | 年份域除以10 | EXTRACT(DECADE from TIMESTAMP '2001-02-16 20:38:40'); | 200 |
| DOW | 每周的星期号(0-6；星期天是0) (仅用于timestamp) | EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40'); | 5 |
| DOY | 一年的第几天(1 -365/366) (仅用于 timestamp) | EXTRACT(DOY from TIMESTAMP '2001-02-16 20:38:40'); | 47 |
| HOUR | 小时域(0-23) | EXTRACT(HOUR from TIMESTAMP '2001-02-16 20:38:40'); | 20 |
| MICROSECONDS | 秒域，包括小数部分，乘以 1,000,000。 | EXTRACT(MICROSECONDS from TIME '17:12:28.5'); | 28500000 |
| MILLENNIUM | 千年 | EXTRACT(MILLENNIUM from TIMESTAMP '2001-02-16 20:38:40'); | 3 |
| MILLISECONDS | 秒域，包括小数部分，乘以 1000。 | EXTRACT(MILLISECONDS from TIME '17:12:28.5'); | 28500 |
| MINUTE | 分钟域(0-59) | EXTRACT(MINUTE from TIMESTAMP '2001-02-16 20:38:40'); | 38 |
| MONTH | 对于timestamp数值，它是一年里的月份数(1-12)；对于interval数值，它是月的数目，然后对12取模(0-11) | EXTRACT(MONTH from TIMESTAMP '2001-02-16 20:38:40'); | 2 |
| QUARTER | 该天所在的该年的季度(1-4)(仅用于 timestamp) | EXTRACT(QUARTER from TIMESTAMP '2001-02-16 20:38:40'); | 1 |
| SECOND | 秒域，包括小数部分(0-59[1]) | EXTRACT(SECOND from TIMESTAMP '2001-02-16 20:38:40'); | 40 |
| WEEK | 该天在所在的年份里是第几周。 | EXTRACT(WEEK from TIMESTAMP '2001-02-16 20:38:40'); | 7 |
| YEAR | 年份域 | EXTRACT(YEAR from TIMESTAMP '2001-02-16 20:38:40'); | 2001 |
4. Current date/time:
We can use the following function to get the current date and/or time:
CURRENT_DATE
CURRENT_TIME
CURRENT_TIMESTAMP
CURRENT_TIME (precision)
CURRENT_TIMESTAMP (precision)
LOCALTIME
LOCALTIMESTAMP
LOCALTIME (precision)
LOCALTIMESTAMP (precision)
All information provided in blog is from PostgreSQL official documentation. The main purpose of this blog post is to facilitate future reference.