9.4. 字串函式及運算子

本節介紹了用於檢查和操作字串的函數和運算子。在這種情況下,字串包括 character、character varying 和 text 型別的值。除非另有說明,否則下面列出的所有函數都可以在這些型別上使用,但是請注意在使用 character 型別時自動空格填充的潛在影響。其中有一些函數還支援對於位元型別的處理。

SQL 定義了一些使用關鍵字而不是逗號分隔參數的字串函數。詳情請見 Table 9.9。PostgreSQL 還提供了使用一般函數呼叫的語法,這些功能的函數版本(請參見 Table 9.10)。

在 PostgreSQL 8.3 之前的版本中,由於存在從這些資料型別到文字的隱式強制轉換,這些函數也將默默接受幾種非字串資料型別的值。這些強制轉換已被刪除,因為它們經常引起令人驚訝的結果。但是,字串連接運算子(||)仍然接受非字串輸入,只要至少一個輸入為字串型別即可,如 Table 9.9 所示。對於其他情況,如果您需要複製以前的行為,請在查詢語句中明確加入型別轉換。

Table 9.9. SQL String Functions and Operators

Function

Return Type

Description

Example

Result

string || string

text

String concatenation

'Post' || 'greSQL'

PostgreSQL

string || non-string or non-string || string

text

String concatenation with one non-string input

'Value: ' || 42

Value: 42

bit_length(string)

int

Number of bits in string

bit_length('jose')

32

char_length(string) or character_length(string)

int

Number of characters in string

char_length('jose')

4

lower(string)

text

Convert string to lower case

lower('TOM')

tom

octet_length(string)

int

Number of bytes in string

octet_length('jose')

4

overlay(string placing string from int [for int])

text

Replace substring

overlay('Txxxxas' placing 'hom' from 2 for 4)

Thomas

position(substring in string)

int

Location of specified substring

position('om' in 'Thomas')

3

substring(string [from int] [for int])

text

Extract substring

substring('Thomas' from 2 for 3)

hom

substring(string from pattern)

text

Extract substring matching POSIX regular expression. See Section 9.7 for more information on pattern matching.

substring('Thomas' from '...$')

mas

substring(string from pattern for escape)

text

Extract substring matching SQL regular expression. See Section 9.7 for more information on pattern matching.

substring('Thomas' from '%#"o_a#"_' for '#')

oma

trim([leading | trailing | both] [characters] from string)

text

Remove the longest string containing only characters from characters (a space by default) from the start, end, or both ends (both is the default) of string

trim(both 'xyz' from 'yxTomxx')

Tom

trim([leading | trailing | both] [from] string [, characters] )

text

Non-standard syntax for trim()

trim(both from 'yxTomxx', 'xyz')

Tom

upper(string)

text

Convert string to upper case

upper('tom')

TOM

其他字串操作的可用函數,在 Table 9.10 中列出。其中一些用於內部實作的SQL標準字符串函數,則在 Table 9.9 中列出。

Table 9.10. Other String Functions

Function

Return Type

Description

Example

Result

ascii(string)

int

ASCII code of the first character of the argument. For UTF8 returns the Unicode code point of the character. For other multibyte encodings, the argument must be an ASCII character.

ascii('x')

120

btrim(string text [, characters text])

text

Remove the longest string consisting only of characters in characters (a space by default) from the start and end of string

btrim('xyxtrimyyx', 'xyz')

trim

chr(int)

text

Character with the given code. For UTF8 the argument is treated as a Unicode code point. For other multibyte encodings the argument must designate an ASCII character. The NULL (0) character is not allowed because text data types cannot store such bytes.

chr(65)

A

concat(str "any" [, str "any" [, ...] ])

text

Concatenate the text representations of all the arguments. NULL arguments are ignored.

concat('abcde', 2, NULL, 22)

abcde222

concat_ws(sep text, str "any" [, str "any" [, ...] ])

text

Concatenate all but the first argument with separators. The first argument is used as the separator string. NULL arguments are ignored.

concat_ws(',', 'abcde', 2, NULL, 22)

abcde,2,22

convert(string bytea, src_encoding name, dest_encoding name)

bytea

Convert string to dest_encoding. The original encoding is specified by src_encoding. The string must be valid in this encoding. Conversions can be defined by CREATE CONVERSION. Also there are some predefined conversions. See Table 9.11 for available conversions.

convert('text_in_utf8', 'UTF8', 'LATIN1')

text_in_utf8 represented in Latin-1 encoding (ISO 8859-1)

convert_from(string bytea, src_encoding name)

text

Convert string to the database encoding. The original encoding is specified by src_encoding. The string must be valid in this encoding.

convert_from('text_in_utf8', 'UTF8')

text_in_utf8 represented in the current database encoding

convert_to(string text, dest_encoding name)

bytea

Convert string to dest_encoding.

convert_to('some text', 'UTF8')

some text represented in the UTF8 encoding

decode(string text, format text)

bytea

Decode binary data from textual representation in string. Options for format are same as in encode.

decode('MTIzAAE=', 'base64')

\x3132330001

encode(data bytea, format text)

text

Encode binary data into a textual representation. Supported formats are: base64, hex, escape. escape converts zero bytes and high-bit-set bytes to octal sequences (\nnn) and doubles backslashes.

encode('123\000\001', 'base64')

MTIzAAE=

format(formatstr text [, formatarg "any" [, ...] ])

text

Format arguments according to a format string. This function is similar to the C function sprintf. See Section 9.4.1.

format('Hello %s, %1$s', 'World')

Hello World, World

initcap(string)

text

Convert the first letter of each word to upper case and the rest to lower case. Words are sequences of alphanumeric characters separated by non-alphanumeric characters.

initcap('hi THOMAS')

Hi Thomas

left(str text, n int)

text

Return first n characters in the string. When n is negative, return all but last |n| characters.

left('abcde', 2)

ab

length(string)

int

Number of characters in string

length('jose')

4

length(string bytea, encoding name )

int

Number of characters in string in the given encoding. The string must be valid in this encoding.

length('jose', 'UTF8')

4

lpad(string text, length int [, fill text])

text

Fill up the string to length length by prepending the characters fill (a space by default). If the string is already longer than length then it is truncated (on the right).

lpad('hi', 5, 'xy')

xyxhi

ltrim(string text [, characters text])

text

Remove the longest string containing only characters from characters (a space by default) from the start of string

ltrim('zzzytest', 'xyz')

test

md5(string)

text

Calculates the MD5 hash of string, returning the result in hexadecimal

md5('abc')

900150983cd24fb0 d6963f7d28e17f72

parse_ident(qualified_identifier text [, strictmode boolean DEFAULT true ] )

text[]

Split qualified_identifier into an array of identifiers, removing any quoting of individual identifiers. By default, extra characters after the last identifier are considered an error; but if the second parameter is false, then such extra characters are ignored. (This behavior is useful for parsing names for objects like functions.) Note that this function does not truncate over-length identifiers. If you want truncation you can cast the result to name[].

parse_ident('"SomeSchema".someTable')

{SomeSchema,sometable}

pg_client_encoding()

name

Current client encoding name

pg_client_encoding()

SQL_ASCII

quote_ident(string text)

text

Return the given string suitably quoted to be used as an identifier in an SQL statement string. Quotes are added only if necessary (i.e., if the string contains non-identifier characters or would be case-folded). Embedded quotes are properly doubled. See also Example 42.1.

quote_ident('Foo bar')

"Foo bar"

quote_literal(string text)

text

Return the given string suitably quoted to be used as a string literal in an SQL statement string. Embedded single-quotes and backslashes are properly doubled. Note that quote_literal returns null on null input; if the argument might be null, quote_nullable is often more suitable. See also Example 42.1.

quote_literal(E'O\'Reilly')

'O''Reilly'

quote_literal(value anyelement)

text

Coerce the given value to text and then quote it as a literal. Embedded single-quotes and backslashes are properly doubled.

quote_literal(42.5)

'42.5'

quote_nullable(string text)

text

Return the given string suitably quoted to be used as a string literal in an SQL statement string; or, if the argument is null, return NULL. Embedded single-quotes and backslashes are properly doubled. See also Example 42.1.

quote_nullable(NULL)

NULL

quote_nullable(value anyelement)

text

Coerce the given value to text and then quote it as a literal; or, if the argument is null, return NULL. Embedded single-quotes and backslashes are properly doubled.

quote_nullable(42.5)

'42.5'

regexp_match(string text, pattern text [, flags text])

text[]

Return captured substring(s) resulting from the first match of a POSIX regular expression to the string. See Section 9.7.3 for more information.

regexp_match('foobarbequebaz', '(bar)(beque)')

{bar,beque}

regexp_matches(string text, pattern text [, flags text])

setof text[]

Return captured substring(s) resulting from matching a POSIX regular expression to the string. See Section 9.7.3 for more information.

regexp_matches('foobarbequebaz', 'ba.', 'g')

{bar}

{baz}(2 rows)

regexp_replace(string text, pattern text, replacement text [, flags text])

text

Replace substring(s) matching a POSIX regular expression. See Section 9.7.3 for more information.

regexp_replace('Thomas', '.[mN]a.', 'M')

ThM

regexp_split_to_array(string text, pattern text [, flags text ])

text[]

Split string using a POSIX regular expression as the delimiter. See Section 9.7.3 for more information.

regexp_split_to_array('hello world', '\s+')

{hello,world}

regexp_split_to_table(string text, pattern text [, flags text])

setof text

Split string using a POSIX regular expression as the delimiter. See Section 9.7.3 for more information.

regexp_split_to_table('hello world', '\s+')

hello

world(2 rows)

repeat(string text, number int)

text

Repeat string the specified number of times

repeat('Pg', 4)

PgPgPgPg

replace(string text, from text, to text)

text

Replace all occurrences in string of substring from with substring to

replace('abcdefabcdef', 'cd', 'XX')

abXXefabXXef

reverse(str)

text

Return reversed string.

reverse('abcde')

edcba

right(str text, n int)

text

Return last n characters in the string. When n is negative, return all but first |n| characters.

right('abcde', 2)

de

rpad(string text, length int [, fill text])

text

Fill up the string to length length by appending the characters fill (a space by default). If the string is already longer than length then it is truncated.

rpad('hi', 5, 'xy')

hixyx

rtrim(string text [, characters text])

text

Remove the longest string containing only characters from characters (a space by default) from the end of string

rtrim('testxxzx', 'xyz')

test

split_part(string text, delimiter text, field int)

text

Split string on delimiter and return the given field (counting from one)

split_part('[email protected][email protected]~ghi', '[email protected]~', 2)

def

strpos(string, substring)

int

Location of specified substring (same as position(substring in string), but note the reversed argument order)

strpos('high', 'ig')

2

substr(string, from [, count])

text

回傳子字串(與 substring(string from from for count) 相同)

substr('alphabet', 3, 2)

ph

starts_with(string, prefix)

bool

Returns true if string starts with prefix.

starts_with('alphabet', 'alph')

t

to_ascii(string text [, encoding text])

text

Convert string to ASCII from another encoding (only supports conversion from LATIN1, LATIN2, LATIN9, and WIN1250 encodings)

to_ascii('Karel')

Karel

to_hex(number int or bigint)

text

Convert number to its equivalent hexadecimal representation

to_hex(2147483647)

7fffffff

translate(string text, from text, to text)

text

Any character in string that matches a character in the from set is replaced by the corresponding character in the to set. If from is longer than to, occurrences of the extra characters in from are removed.

translate('12345', '143', 'ax')

a2x5

concat、concat_ws 和 format 函數是動態參數,因此可以將要連接或格式化的值以 VARIADIC 關鍵字標記的陣列(請參閱第 37.5.5 節)輸入。 將陣列的元素視為函數的一個普通參數。如果動態參數陣列參數為 NULL,則 concat 和 concat_ws 回傳 NULL,但是 format 將 NULL 視為零元素陣列。

另請參閱第 9.20 節中的彙總函數 string_agg。

Table 9.11. Built-in Conversions

Conversion Name

Source Encoding

Destination Encoding

ascii_to_mic

SQL_ASCII

MULE_INTERNAL

ascii_to_utf8

SQL_ASCII

UTF8

big5_to_euc_tw

BIG5

EUC_TW

big5_to_mic

BIG5

MULE_INTERNAL

big5_to_utf8

BIG5

UTF8

euc_cn_to_mic

EUC_CN

MULE_INTERNAL

euc_cn_to_utf8

EUC_CN

UTF8

euc_jp_to_mic

EUC_JP

MULE_INTERNAL

euc_jp_to_sjis

EUC_JP

SJIS

euc_jp_to_utf8

EUC_JP

UTF8

euc_kr_to_mic

EUC_KR

MULE_INTERNAL

euc_kr_to_utf8

EUC_KR

UTF8

euc_tw_to_big5

EUC_TW

BIG5

euc_tw_to_mic

EUC_TW

MULE_INTERNAL

euc_tw_to_utf8

EUC_TW

UTF8

gb18030_to_utf8

GB18030

UTF8

gbk_to_utf8

GBK

UTF8

iso_8859_10_to_utf8

LATIN6

UTF8

iso_8859_13_to_utf8

LATIN7

UTF8

iso_8859_14_to_utf8

LATIN8

UTF8

iso_8859_15_to_utf8

LATIN9

UTF8

iso_8859_16_to_utf8

LATIN10

UTF8

iso_8859_1_to_mic

LATIN1

MULE_INTERNAL

iso_8859_1_to_utf8

LATIN1

UTF8

iso_8859_2_to_mic

LATIN2

MULE_INTERNAL

iso_8859_2_to_utf8

LATIN2

UTF8

iso_8859_2_to_windows_1250

LATIN2

WIN1250

iso_8859_3_to_mic

LATIN3

MULE_INTERNAL

iso_8859_3_to_utf8

LATIN3

UTF8

iso_8859_4_to_mic

LATIN4

MULE_INTERNAL

iso_8859_4_to_utf8

LATIN4

UTF8

iso_8859_5_to_koi8_r

ISO_8859_5

KOI8R

iso_8859_5_to_mic

ISO_8859_5

MULE_INTERNAL

iso_8859_5_to_utf8

ISO_8859_5

UTF8

iso_8859_5_to_windows_1251

ISO_8859_5

WIN1251

iso_8859_5_to_windows_866

ISO_8859_5

WIN866

iso_8859_6_to_utf8

ISO_8859_6

UTF8

iso_8859_7_to_utf8

ISO_8859_7

UTF8

iso_8859_8_to_utf8

ISO_8859_8

UTF8

iso_8859_9_to_utf8

LATIN5

UTF8

johab_to_utf8

JOHAB

UTF8

koi8_r_to_iso_8859_5

KOI8R

ISO_8859_5

koi8_r_to_mic

KOI8R

MULE_INTERNAL

koi8_r_to_utf8

KOI8R

UTF8

koi8_r_to_windows_1251

KOI8R

WIN1251

koi8_r_to_windows_866

KOI8R

WIN866

koi8_u_to_utf8

KOI8U

UTF8

mic_to_ascii

MULE_INTERNAL

SQL_ASCII

mic_to_big5

MULE_INTERNAL

BIG5

mic_to_euc_cn

MULE_INTERNAL

EUC_CN

mic_to_euc_jp

MULE_INTERNAL

EUC_JP

mic_to_euc_kr

MULE_INTERNAL

EUC_KR

mic_to_euc_tw

MULE_INTERNAL

EUC_TW

mic_to_iso_8859_1

MULE_INTERNAL

LATIN1

mic_to_iso_8859_2

MULE_INTERNAL

LATIN2

mic_to_iso_8859_3

MULE_INTERNAL

LATIN3

mic_to_iso_8859_4

MULE_INTERNAL

LATIN4

mic_to_iso_8859_5

MULE_INTERNAL

ISO_8859_5

mic_to_koi8_r

MULE_INTERNAL

KOI8R

mic_to_sjis

MULE_INTERNAL

SJIS

mic_to_windows_1250

MULE_INTERNAL

WIN1250

mic_to_windows_1251

MULE_INTERNAL

WIN1251

mic_to_windows_866

MULE_INTERNAL

WIN866

sjis_to_euc_jp

SJIS

EUC_JP

sjis_to_mic

SJIS

MULE_INTERNAL

sjis_to_utf8

SJIS

UTF8

windows_1258_to_utf8

WIN1258

UTF8

uhc_to_utf8

UHC

UTF8

utf8_to_ascii

UTF8

SQL_ASCII

utf8_to_big5

UTF8

BIG5

utf8_to_euc_cn

UTF8

EUC_CN

utf8_to_euc_jp

UTF8

EUC_JP

utf8_to_euc_kr

UTF8

EUC_KR

utf8_to_euc_tw

UTF8

EUC_TW

utf8_to_gb18030

UTF8

GB18030

utf8_to_gbk

UTF8

GBK

utf8_to_iso_8859_1

UTF8

LATIN1

utf8_to_iso_8859_10

UTF8

LATIN6

utf8_to_iso_8859_13

UTF8

LATIN7

utf8_to_iso_8859_14

UTF8

LATIN8

utf8_to_iso_8859_15

UTF8

LATIN9

utf8_to_iso_8859_16

UTF8

LATIN10

utf8_to_iso_8859_2

UTF8

LATIN2

utf8_to_iso_8859_3

UTF8

LATIN3

utf8_to_iso_8859_4

UTF8

LATIN4

utf8_to_iso_8859_5

UTF8

ISO_8859_5

utf8_to_iso_8859_6

UTF8

ISO_8859_6

utf8_to_iso_8859_7

UTF8

ISO_8859_7

utf8_to_iso_8859_8

UTF8

ISO_8859_8

utf8_to_iso_8859_9

UTF8

LATIN5

utf8_to_johab

UTF8

JOHAB

utf8_to_koi8_r

UTF8

KOI8R

utf8_to_koi8_u

UTF8

KOI8U

utf8_to_sjis

UTF8

SJIS

utf8_to_windows_1258

UTF8

WIN1258

utf8_to_uhc

UTF8

UHC

utf8_to_windows_1250

UTF8

WIN1250

utf8_to_windows_1251

UTF8

WIN1251

utf8_to_windows_1252

UTF8

WIN1252

utf8_to_windows_1253

UTF8

WIN1253

utf8_to_windows_1254

UTF8

WIN1254

utf8_to_windows_1255

UTF8

WIN1255

utf8_to_windows_1256

UTF8

WIN1256

utf8_to_windows_1257

UTF8

WIN1257

utf8_to_windows_866

UTF8

WIN866

utf8_to_windows_874

UTF8

WIN874

windows_1250_to_iso_8859_2

WIN1250

LATIN2

windows_1250_to_mic

WIN1250

MULE_INTERNAL

windows_1250_to_utf8

WIN1250

UTF8

windows_1251_to_iso_8859_5

WIN1251

ISO_8859_5

windows_1251_to_koi8_r

WIN1251

KOI8R

windows_1251_to_mic

WIN1251

MULE_INTERNAL

windows_1251_to_utf8

WIN1251

UTF8

windows_1251_to_windows_866

WIN1251

WIN866

windows_1252_to_utf8

WIN1252

UTF8

windows_1256_to_utf8

WIN1256

UTF8

windows_866_to_iso_8859_5

WIN866

ISO_8859_5

windows_866_to_koi8_r

WIN866

KOI8R

windows_866_to_mic

WIN866

MULE_INTERNAL

windows_866_to_utf8

WIN866

UTF8

windows_866_to_windows_1251

WIN866

WIN

windows_874_to_utf8

WIN874

UTF8

euc_jis_2004_to_utf8

EUC_JIS_2004

UTF8

utf8_to_euc_jis_2004

UTF8

EUC_JIS_2004

shift_jis_2004_to_utf8

SHIFT_JIS_2004

UTF8

utf8_to_shift_jis_2004

UTF8

SHIFT_JIS_2004

euc_jis_2004_to_shift_jis_2004

EUC_JIS_2004

SHIFT_JIS_2004

shift_jis_2004_to_euc_jis_2004

SHIFT_JIS_2004

EUC_JIS_2004

轉換名稱遵循標準的命名規則:來源編碼的正式名稱,所有非字母數字字元均用下底線代替,接在 _to_ 之後,然後是經過類似處理的目標編碼名稱。因此,名稱可能與習慣的編碼名稱有所不同。

9.4.1. format

The function format produces output formatted according to a format string, in a style similar to the C function sprintf.

format(formatstr text [, formatarg "any" [, ...] ])

formatstr is a format string that specifies how the result should be formatted. Text in the format string is copied directly to the result, except where format specifiers are used. Format specifiers act as placeholders in the string, defining how subsequent function arguments should be formatted and inserted into the result. Each formatarg argument is converted to text according to the usual output rules for its data type, and then formatted and inserted into the result string according to the format specifier(s).

Format specifiers are introduced by a % character and have the form

%[position][flags][width]type

where the component fields are:position (optional)

A string of the form n$ where n is the index of the argument to print. Index 1 means the first argument after formatstr. If the position is omitted, the default is to use the next argument in sequence.flags (optional)

Additional options controlling how the format specifier's output is formatted. Currently the only supported flag is a minus sign (-) which will cause the format specifier's output to be left-justified. This has no effect unless the width field is also specified.width (optional)

Specifies the minimum number of characters to use to display the format specifier's output. The output is padded on the left or right (depending on the - flag) with spaces as needed to fill the width. A too-small width does not cause truncation of the output, but is simply ignored. The width may be specified using any of the following: a positive integer; an asterisk (*) to use the next function argument as the width; or a string of the form *n$ to use the nth function argument as the width.

If the width comes from a function argument, that argument is consumed before the argument that is used for the format specifier's value. If the width argument is negative, the result is left aligned (as if the - flag had been specified) within a field of length abs(width).type (required)

The type of format conversion to use to produce the format specifier's output. The following types are supported:

  • s formats the argument value as a simple string. A null value is treated as an empty string.

  • I treats the argument value as an SQL identifier, double-quoting it if necessary. It is an error for the value to be null (equivalent to quote_ident).

  • L quotes the argument value as an SQL literal. A null value is displayed as the string NULL, without quotes (equivalent to quote_nullable).

In addition to the format specifiers described above, the special sequence %% may be used to output a literal % character.

Here are some examples of the basic format conversions:

SELECT format('Hello %s', 'World');
Result: Hello World
SELECT format('Testing %s, %s, %s, %%', 'one', 'two', 'three');
Result: Testing one, two, three, %
SELECT format('INSERT INTO %I VALUES(%L)', 'Foo bar', E'O\'Reilly');
Result: INSERT INTO "Foo bar" VALUES('O''Reilly')
SELECT format('INSERT INTO %I VALUES(%L)', 'locations', 'C:\Program Files');
Result: INSERT INTO locations VALUES('C:\Program Files')

Here are examples using width fields and the - flag:

SELECT format('|%10s|', 'foo');
Result: | foo|
SELECT format('|%-10s|', 'foo');
Result: |foo |
SELECT format('|%*s|', 10, 'foo');
Result: | foo|
SELECT format('|%*s|', -10, 'foo');
Result: |foo |
SELECT format('|%-*s|', 10, 'foo');
Result: |foo |
SELECT format('|%-*s|', -10, 'foo');
Result: |foo |

These examples show use of position fields:

SELECT format('Testing %3$s, %2$s, %1$s', 'one', 'two', 'three');
Result: Testing three, two, one
SELECT format('|%*2$s|', 'foo', 10, 'bar');
Result: | bar|
SELECT format('|%1$*2$s|', 'foo', 10, 'bar');
Result: | foo|

Unlike the standard C function sprintf, PostgreSQL's format function allows format specifiers with and without position fields to be mixed in the same format string. A format specifier without a position field always uses the next argument after the last argument consumed. In addition, the format function does not require all function arguments to be used in the format string. For example:

SELECT format('Testing %3$s, %2$s, %s', 'one', 'two', 'three');
Result: Testing three, two, three

The %I and %L format specifiers are particularly useful for safely constructing dynamic SQL statements. See Example 42.1.

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