9.5. 位元字串函式及運算子

This section describes functions and operators for examining and manipulating values of typebytea.

SQLdefines some string functions that use key words, rather than commas, to separate arguments. Details are inTable 9.11.PostgreSQLalso provides versions of these functions that use the regular function invocation syntax (seeTable 9.12).

Note

The sample results shown on this page assume that the server parameterbytea_outputis set toescape(the traditional PostgreSQL format).

Table 9.11. SQLBinary String Functions and Operators

Function	Return Type	Description	Example	Result
string`		_string`_	bytea	String concatenation	`E'\\Post'::bytea		E'\047gres\000'::bytea`	\\Post'gres\000
octet_length(string)	int	Number of bytes in binary string	octet_length(E'jo\\000se'::bytea)	5
overlay(string_placingstring_fromint[forint])	bytea	Replace substring	overlay(E'Th\\000omas'::bytea placing E'\\002\\003'::bytea from 2 for 3)	T\\002\\003mas
position(substring_instring_)	int	Location of specified substring	position(E'\\000om'::bytea in E'Th\\000omas'::bytea)	3
substring(string[fromint] [forint])	bytea	Extract substring	substring(E'Th\\000omas'::bytea from 2 for 3)	h\000o
trim([both]bytes_fromstring_)	bytea	Remove the longest string containing only bytes appearing inbytes_from the start and end ofstring_	trim(E'\\000\\001'::bytea from E'\\000Tom\\001'::bytea)	Tom

Additional binary string manipulation functions are available and are listed inTable 9.12. Some of them are used internally to implement theSQL-standard string functions listed inTable 9.11.

Table 9.12. Other Binary String Functions

Function	Return Type	Description	Example	Result
btrim(stringbytea,bytesbytea)	bytea	Remove the longest string containing only bytes appearing inbytes_from the start and end ofstring_	btrim(E'\\000trim\\001'::bytea, E'\\000\\001'::bytea)	trim
decode(stringtext,formattext)	bytea	Decode binary data from textual representation instring. Options for_format_are same as inencode.	decode(E'123\\000456', 'escape')	123\000456
encode(databytea,formattext)	text	Encode binary data into a textual representation. Supported formats are:base64,hex,escape.escapeconverts zero bytes and high-bit-set bytes to octal sequences (\nnn) and doubles backslashes.	encode(E'123\\000456'::bytea, 'escape')	123\000456
get_bit(string,offset)	int	Extract bit from string	get_bit(E'Th\\000omas'::bytea, 45)	1
get_byte(string,offset)	int	Extract byte from string	get_byte(E'Th\\000omas'::bytea, 4)	109
length(string)	int	Length of binary string	length(E'jo\\000se'::bytea)	5
md5(string)	text	Calculates the MD5 hash ofstring, returning the result in hexadecimal	md5(E'Th\\000omas'::bytea)	8ab2d3c9689aaf18 b4958c334c82d8b1
set_bit(string,offset,newvalue)	bytea	Set bit in string	set_bit(E'Th\\000omas'::bytea, 45, 0)	Th\000omAs
set_byte(string,offset,newvalue)	bytea	Set byte in string	set_byte(E'Th\\000omas'::bytea, 4, 64)	Th\000o@as

get_byteandset_bytenumber the first byte of a binary string as byte 0.get_bitandset_bitnumber bits from the right within each byte; for example bit 0 is the least significant bit of the first byte, and bit 15 is the most significant bit of the second byte.

See also the aggregate functionstring_agginSection 9.20and the large object functions inSection 34.4.