postgresql 常用函数汇总

函数：string || string 
说明：String concatenation 字符串连接操作
例子：'Post' || 'greSQL' ＝ PostgreSQL

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函数：string || non-string or non-string || string
说明：String concatenation with one non-string input 字符串与非字符串类型进行连接操作
例子：'Value: ' || 42 = Value: 42

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函数：bit_length(string)
说明：Number of bits in string 计算字符串的位数
例子：bit_length('jose') = 32

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函数：char_length(string) or character_length(string)
说明：Number of characters in string 计算字符串中字符个数
例子：char_length('jose') = 4

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函数：lower(string)
说明：Convert string to lower case 转换字符串为小写
例子：bit_length('jose') = 32

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函数：octet_length(string)
说明：Number of bytes in string 计算字符串的字节数
例子：octet_length('jose') = 4

函数：overlay(string placing string from int [for int])
说明：Replace substring 替换字符串中任意长度的子字串为新字符串
例子：overlay('Txxxxas' placing 'hom' from 2 for 4) = 4

函数：position(substring in string)
说明：Location of specified substring 子串在一字符串中的位置
例子：position('om' in 'Thomas') = 3

函数：substring(string [from int] [for int])
说明：Extract substring 截取任意长度的子字符串
例子：substring('Thomas' from 2 for 3) = hom

函数：substring(string from pattern)
说明：Extract substring matching POSIX regular expression. See Section 9.7 for more information on pattern matching. 利用正则表达式对一字符串进行任意长度的字串的截取
例子：substring('Thomas' from '...$') = mas

函数：substring(string from pattern for escape)
说明：Extract substring matching SQL regular expression. See Section 9.7 for more information on pattern matching. 利于正则表达式对某类字符进行删除，以得到子字符串
例子：trim(both 'x' from 'xTomxx') = Tom

函数：trim([leading | trailing | both] [characters] from string)
说明：Remove the longest string containing only the characters (a space by default) from the start/end/both ends of the string 去除尽可能长开始，结束或者两边的某类字符，默认为去除空白字符，当然可以自己指定，可同时指定多个要删除的字符串
例子：trim(both 'x' from 'xTomxx') = Tom

函数：upper(string)
说明：Convert string to uppercase 将字符串转换为大写
例子：upper('tom') = TOM

函数：ascii(string)
说明：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 a strictly ASCII character. 得到某一个字符的Assii值
例子：ascii('x') = 120

函数：btrim(string text [, characters text])
说明：Remove the longest string consisting only of characters in characters (a space by default) from the start and end of string 去除字符串两边的所有指定的字符，可同时指定多个字符
例子：btrim('xyxtrimyyx', 'xy') = trim

函数：chr(int)
说明：Character with the given code. For UTF8 the argument is treated as a Unicode code point. For other multibyte encodings the argument must designate a strictly ASCII character. The NULL (0) character is not allowed because text data types cannot store such bytes. 得到某ACSII值对应的字符
例子：chr(65) = A

函数：convert(string bytea, src_encoding name, dest_encoding name)
说明：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 9-7 for available conversions. 转换字符串编码，指定源编码与目标编码
例子：convert('text_in_utf8', 'UTF8', 'LATIN1') = text_in_utf8 represented in ISO 8859-1 encoding

函数：convert_from(string bytea, src_encoding name)
说明：Convert string to the database encoding. The original encoding is specified by src_encoding. The string must be valid in this encoding. 转换字符串编码，自己要指定源编码,目标编码默认为数据库指定编码，
例子：convert_from('text_in_utf8', 'UTF8') = text_in_utf8 represented in the current database encoding

函数：convert_to(string text, dest_encoding name)
说明：Convert string to dest_encoding.转换字符串编码，源编码默认为数据库指定编码，自己要指定目标编码,
例子：convert_to('some text', 'UTF8') = some text represented in the UTF8 encoding

函数：decode(string text, type text)
说明：Decode binary data from string previously encoded with encode. Parameter type is same as in encode. 对字符串按指定的类型进行解码
例子：decode('MTIzAAE=', 'base64') = 123\000\001

函数：encode(data bytea, type text)
说明：Encode binary data to different representation. Supported types are: base64, hex, escape. Escape merely outputs null bytes as \000 and doubles backslashes. 与decode相反，对字符串按指定类型进行编码
例子：encode(E'123\\000\\001', 'base64') = MTIzAAE=

函数：initcap(string)
说明：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. 将字符串所有的单词进行格式化，首字母大写，其它为小写
例子：initcap('hi THOMAS') = Hi Thomas

函数：length(string)
说明：Number of characters in string 讲算字符串长度
例子：length('jose') = 4

函数：length(stringbytea, encoding name )
说明：Number of characters in string in the given encoding. The string must be valid in this encoding. 计算字符串长度，指定字符串使用的编码
例子：length('jose', 'UTF8') = 4

函数：lpad(string text, length int [, fill text])
说明：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). 对字符串左边进行某类字符自动填充，即不足某一长度，则在左边自动补上指定的字符串，直至达到指定长度，可同时指定多个自动填充的字符
例子：lpad('hi', 5, 'xy') = xyxhi

函数：ltrim(string text [, characters text])
说明：Remove the longest string containing only characters from characters (a space by default) from the start of string 删除字符串左边某一些的字符，可以时指定多个要删除的字符
例子：trim

函数：md5(string)
说明：Calculates the MD5 hash of string, returning the result in hexadecimal 将字符串进行md5编码
例子：md5('abc') = 900150983cd24fb0 d6963f7d28e17f72

函数：pg_client_encoding()
说明：Current client encoding name 得到pg客户端编码
例子：pg_client_encoding() = SQL_ASCII

函数：quote_ident(string text)
说明：Return the given string suitably quoted to be used as an identifier in an SQL statement string. Quotes are added only if