字符串连接
SELECT 'A'||'B'; 字符串连接，适用于字符串与数字连接
SELECT CONCAT(10,'a','hello',20.0); 连接所有参数，个数不限，类型不限
字母转换
SELECT LOWER('ABCD'); 将字符转换成小写
SELECT UPPER('ABCD'); 将字符转换成大写
SELECT INITCAP('abcd'); 将字符串中每个单词的首字母大写
字符串替换
SELECT REPLACE('aaaxxx','a','_'); 替换字符串：SELECT REPLACE('aaaxxx','a','_');
SELECT TRIM(' asd '); 删除头尾部空格 → TRIM(BOTH ' asd ')
SELECT TRIM(LEADING ' asd '),LENGTH(TRIM(LEADING ' asd ')); 删除头部空格
SELECT TRIM(TRAILING ' asd '),LENGTH(TRIM(TRAILING ' asd ')); 删除尾部空格
SELECT TRIM('aaahelloworldaaa','a'); 可以指定删除字符，默认删除空格
字符串查找与截取
SELECT POSITION('lo' in 'hello'); 给定子字符串在字符串的位置，查询不到时返回0。
SELECT SUBSTRING('hello world' FROM 3 FOR 10); 截取字符串，从from位置截取长度for
SELECT LENGTH('ABCD'); 字符串中字符的个数
SELECT LEFT('HELLO',2); 返回字符串前n个字符，n为负数时返回除最后|n|个字符以外的所有字符
SELECT RIGHT('HELLO',2); 返回字符串后n个字符，n为负数时返回除最前|n|个字符意外的所有字符
strpos(string, substring)；例如：strpos('high','ig')，返回2
字符串类型转换
SELECT CAST(10.0 AS text);
SELECT CAST(salary AS int) FROM company3; 类型转换
字符串判断null
COALESCE(b.price, 其它值)
字符串前缀填充
select lpad(‘256’, 6, ‘0’) //256不满6位数时，在左边补0，最终达到6位长度
字符串后缀填充
select rpad(‘256’, 6, ‘0’) //256不满6位数时，在右边补0，最终达到6位长度
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函数：string || string
说明：String concatenation 字符串连接操作
例子：'Post' || 'greSQL' ＝ PostgreSQL
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
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
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
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
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 necessary (i.e., if the string contains non-identifier characters or would be case-folded). Embedded quotes are properly doubled. 对某一字符串加上两引号
例子：quote_ident('Foo bar') = "Foo bar"
函数：quote_literal(string text)
说明：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. 对字符串里两边加上单引号，如果字符串里面出现sql编码的单个单引号，则会被表达成两个单引号
例子：quote_literal('O\'Reilly') = 'O''Reilly'
函数：quote_literal(value anyelement)
说明：Coerce the given value to text and then quote it as a literal. Embedded single-quotes and backslashes are properly doubled. 将一数值转换为字符串，并为其两边加上单引号，如果数值中间出现了单引号，也会被表示成两个单引号
例子：quote_literal(42.5) = '42.5'
函数：regexp_matches(string text, pattern text [, flags text])
说明：Return all captured substrings resulting from matching a POSIX regular expression against the string. See Section 9.7.3 for more information. 对字符串按正则表达式进行匹配，如果存在则会在结果数组中表示出来
例子：regexp_matches('foobarbequebaz', '(bar)(beque)') = {bar,beque}
函数：regexp_replace(string text, pattern text, replacement text [, flags text])
说明：Replace substring(s) matching a POSIX regular expression. See Section 9.7.3 for more information. 利用正则表达式对字符串进行替换
例子：regexp_replace('Thomas', '.[mN]a.', 'M') = ThM
函数：regexp_split_to_array(string text, pattern text [, flags text ])
说明：Split string using a POSIX regular expression as the delimiter. See Section 9.7.3 for more information. 利用正则表达式将字符串分割成数组
例子：regexp_split_to_array('hello world', E'\\s+') = {hello,world}
函数：regexp_split_to_table(string text, pattern text [, flags text])
说明：Split string using a POSIX regular expression as the delimiter. See Section 9.7.3 for more information. 利用正则表达式将字符串分割成表格
例子：regexp_split_to_table('hello world', E'\\s+') =
hello
world
(2 rows)
函数：repeat(string text, number int)
说明：Repeat string the specified number of times 重复字符串一指定次数
例子：repeat('Pg', 4) = PgPgPgPg
函数：replace(string text, from text, to text)
说明：Replace all occurrences in string of substring from with substring to 将字符的某一子串替换成另一子串
例子：('abcdefabcdef', 'cd', 'XX') = abXXefabXXef
函数：rpad(string text, length int [, fill text])
说明：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. 对字符串进行填充，填充内容为指定的字符串
例子：rpad('hi', 5, 'xy') = hixyx
函数：rtrim(string text [, characters text])
说明：Remove the longest string containing only characters from characters (a space by default) from the end of string
去除字符串右边指定的字符
例子：rtrim('trimxxxx', 'x') = trim
函数：split_part(string text, delimiter text, field int)
说明：Split string on delimiter and return the given field (counting from one) 对字符串按指定子串进行分割，并返回指定的数值位置的值
例子：split_part('abc~@~def~@~ghi', '~@~', 2) = def
函数：strpos(string, substring)
说明：Location of specified substring (same as position(substring in string), but note the reversed argument order) 指定字符串在目标字符串的位置
例子：strpos('high', 'ig') = 2
函数：substr(string, from [, count])
说明：Extract substring (same as substring(string from from for count)) 截取子串
例子：substr('alphabet', 3, 2) = ph
函数：to_ascii(string text [, encoding text])
说明：Convert string to ASCII from another encoding (only supports conversion from LATIN1, LATIN2, LATIN9, and WIN1250 encodings) 将字符串转换成ascii编码字符串
例子：to_ascii('Karel') = Karel
函数：to_hex(number int or bigint)
说明：Convert number to its equivalent hexadecimal representation 对数值进行十六进制编码
例子：to_hex(2147483647) = 7fffffff
函数：translate(string text, from text, to text)
说明：Any character in string that matches a character in the from set is replaced by the corresponding character in the to set 将字符串中某些匹配的字符替换成指定字符串，目标字符与源字符都可以同时指定多个
例子：translate('12345', '14', 'ax') = a23x5
