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。
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
substring('Thomas' from '...$')
mas
substring(string from pattern for escape)
text
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。
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('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('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
quote_ident('Foo bar')
"Foo bar"
quote_literal(string text)
text
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
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[]
regexp_match('foobarbequebaz', '(bar)(beque)')
{bar,beque}
regexp_matches(string text, pattern text [, flags text])
setof text[]
regexp_matches('foobarbequebaz', 'ba.', 'g')
{bar}
{baz}(2 rows)
regexp_replace(string text, pattern text, replacement text [, flags text])
text
regexp_replace('Thomas', '.[mN]a.', 'M')
ThM
regexp_split_to_array(string text, pattern text [, flags text ])
text[]
regexp_split_to_array('hello world', '\s+')
{hello,world}
regexp_split_to_table(string text, pattern text [, flags text])
setof text
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('abc~@~def~@~ghi', '~@~', 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。
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。
formatThe function format produces output formatted according to a format string, in a style similar to the C function sprintf.
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
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 _n_th 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:
Here are examples using width fields and the - flag:
These examples show use of position fields:
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:
The %I and %L format specifiers are particularly useful for safely constructing dynamic SQL statements. See Example 42.1.
Extract substring matching POSIX regular expression. See for more information on pattern matching.
Extract substring matching SQL regular expression. See for more information on pattern matching.
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 for available conversions.
Format arguments according to a format string. This function is similar to the C function sprintf. See .
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 .
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 .
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 .
Return captured substring(s) resulting from the first match of a POSIX regular expression to the string. See for more information.
Return captured substring(s) resulting from matching a POSIX regular expression to the string. See for more information.
Replace substring(s) matching a POSIX regular expression. See for more information.
Split string using a POSIX regular expression as the delimiter. See for more information.
Split string using a POSIX regular expression as the delimiter. See for more information.