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9.4. String Functions and Operators

This section describes functions and operators for examining and manipulating string values. Strings in this context include values of all the types character, character varying, and text. Unless otherwise noted, all of the functions listed below work on all of these types, but be wary of potential effects of the automatic padding when using the character type. Generally, the functions described here also work on data of non-string types by converting that data to a string representation first. Some functions also exist natively for the bit-string types.

SQL defines some string functions with a special syntax where certain key words rather than commas are used to separate the arguments. Details are in Table 9-5. These functions are also implemented using the regular syntax for function invocation. (See Table 9-6.)

Table 9-5. SQL String Functions and Operators

Function Return Type Description Example Result
string || string text String concatenation 'Post' || 'greSQL' PostgreSQL
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
convert(string using conversion_name) text Change encoding using specified conversion name. Conversions can be defined by CREATE CONVERSION. Also there are some pre-defined conversion names. See Table 9-7 for available conversion names. convert('PostgreSQL' using iso_8859_1_to_utf8) 'PostgreSQL' in UTF8 (Unicode, 8-bit) encoding
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 the characters (a space by default) from the start/end/both ends of the string trim(both 'x' from 'xTomxx') Tom
upper(string) text Convert string to uppercase upper('tom') TOM

Additional string manipulation functions are available and are listed in Table 9-6. Some of them are used internally to implement the SQL-standard string functions listed in Table 9-5.

Table 9-6. Other String Functions

Function Return Type Description Example Result
ascii(text) int ASCII code of the first character of the argument 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', 'xy') trim
chr(int) text Character with the given ASCII code chr(65) A
convert(string text, [src_encoding name,] dest_encoding name) text Convert string to dest_encoding. The original encoding is specified by src_encoding. If src_encoding is omitted, database encoding is assumed. convert( 'text_in_utf8', 'UTF8', 'LATIN1') text_in_utf8 represented in ISO 8859-1 encoding
decode(string text, type text) bytea Decode binary data from string previously encoded with encode. Parameter type is same as in encode. decode('MTIzAAE=', 'base64') 123\000\001
encode(data bytea, type text) text Encode binary data to ASCII-only representation. Supported types are: base64, hex, escape. encode( '123\\000\\001', 'base64') MTIzAAE=
initcap(text) text Convert the first letter of each word to uppercase and the rest to lowercase. Words are sequences of alphanumeric characters separated by non-alphanumeric characters. initcap('hi THOMAS') Hi Thomas
length(string text) int Number of characters in string length('jose') 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('zzzytrim', 'xyz') trim
md5(string text) text Calculates the MD5 hash of string, returning the result in hexadecimal md5('abc') 900150983cd24fb0 d6963f7d28e17f72
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. 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 quotes and backslashes are properly doubled. quote_literal( 'O\'Reilly') 'O''Reilly'
regexp_replace(string text, pattern text, replacement text [,flags text]) text Replace substring matching POSIX regular expression. See Section 9.7 for more information on pattern matching. regexp_replace('Thomas', '.[mN]a.', 'M') ThM
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
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('trimxxxx', 'x') trim
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 Extract substring (same as substring(string from from for count)) substr('alphabet', 3, 2) ph
to_ascii(text [, encoding]) text Convert text to ASCII from another encoding [a] 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 translate('12345', '14', 'ax') a23x5
Notes:
a. The to_ascii function supports conversion from LATIN1, LATIN2, LATIN9, and WIN1250 encodings only.

Table 9-7. Built-in Conversions

Conversion Name [a] 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 KOI8
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 KOI8 ISO_8859_5
koi8_r_to_mic KOI8 MULE_INTERNAL
koi8_r_to_utf8 KOI8 UTF8
koi8_r_to_windows_1251 KOI8 WIN1251
koi8_r_to_windows_866 KOI8 WIN866
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 KOI8
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
tcvn_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 KOI8
utf8_to_sjis UTF8 SJIS
utf8_to_tcvn 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_1256 UTF8 WIN1256
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 KOI8
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 KOI8
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
Notes:
a. The conversion names follow a standard naming scheme: The official name of the source encoding with all non-alphanumeric characters replaced by underscores followed by _to_ followed by the equally processed destination encoding name. Therefore the names might deviate from the customary encoding names.

Comments


Sept. 25, 2006, 9:03 a.m.

How to create an eval function.

1) Install plpgsql
2) create the following function :

CREATE OR REPLACE FUNCTION eval(varchar)
RETURNS numeric AS
$BODY$
DECLARE
s varchar;
r numeric;
BEGIN

s := 'select ' || $1 ;
execute s into r;
return r;

END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;

Then you can compute mathematical string expression :

select eval('3*6') will give 18
select eval('(3+6)*10+1+cos(0)') will give 92

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