mysql自带的字符串相关函数

转自mysql帮助文档

String Functions

String-valued functions return NULL if the length of the result would be greater than the value of the max_allowed_packet system variable. See Section 7.5.2, “Tuning Server Parameters”.

For functions that operate on string positions, the first position is numbered 1.

• ASCII(str)

  Returns the numeric value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL. ASCII() works for characters with numeric values from 0 to 255.

  mysql> SELECT ASCII('2');
          -> 50
  mysql> SELECT ASCII(2);
          -> 50
  mysql> SELECT ASCII('dx');
          -> 100
  

  See also the ORD() function.

• BIN(N)

  Returns a string representation of the binary value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,2). Returns NULL if N is NULL.

  mysql> SELECT BIN(12);
          -> '1100'
  

• BIT_LENGTH(str)

  Returns the length of the string str in bits.

  mysql> SELECT BIT_LENGTH('text');
          -> 32
  

• CHAR(N,... [USING charset_name])

  CHAR() interprets each argument N as an integer and returns a string consisting of the characters given by the code values of those integers. NULL values are skipped.

  mysql> SELECT CHAR(77,121,83,81,'76');
          -> 'MySQL'
  mysql> SELECT CHAR(77,77.3,'77.3');
          -> 'MMM'
  

  CHAR() arguments larger than 255 are converted into multiple result bytes. For example, CHAR(256) is equivalent to CHAR(1,0), and CHAR(256*256) is equivalent to CHAR(1,0,0):

  mysql> SELECT HEX(CHAR(1,0)), HEX(CHAR(256));
  +----------------+----------------+
  | HEX(CHAR(1,0)) | HEX(CHAR(256)) |
  +----------------+----------------+
  | 0100           | 0100           |
  +----------------+----------------+
  mysql> SELECT HEX(CHAR(1,0,0)), HEX(CHAR(256*256));
  +------------------+--------------------+
  | HEX(CHAR(1,0,0)) | HEX(CHAR(256*256)) |
  +------------------+--------------------+
  | 010000           | 010000             |
  +------------------+--------------------+
  

  By default, CHAR() returns a binary string. To produce a string in a given character set, use the optional USING clause:

  mysql> SELECT CHARSET(CHAR(0x65)), CHARSET(CHAR(0x65 USING utf8));
  +---------------------+--------------------------------+
  | CHARSET(CHAR(0x65)) | CHARSET(CHAR(0x65 USING utf8)) |
  +---------------------+--------------------------------+
  | binary              | utf8                           |
  +---------------------+--------------------------------+
  

  If USING is given and the result string is illegal for the given character set, a warning is issued. Also, if strict SQL mode is enabled, the result from CHAR() becomes NULL.

• CHAR_LENGTH(str)

  Returns the length of the string str, measured in characters. A multi-byte character counts as a single character. This means that for a string containing five two-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5.

• CHARACTER_LENGTH(str)

  CHARACTER_LENGTH() is a synonym for CHAR_LENGTH().

• CONCAT(str1,str2,...)

  Returns the string that results from concatenating the arguments. May have one or more arguments. If all arguments are non-binary strings, the result is a non-binary string. If the arguments include any binary strings, the result is a binary string. A numeric argument is converted to its equivalent binary string form; if you want to avoid that, you can use an explicit type cast, as in this example:

  SELECT CONCAT(CAST(int_col AS CHAR), char_col);
  

  CONCAT() returns NULL if any argument is NULL.

  mysql> SELECT CONCAT('My', 'S', 'QL');
          -> 'MySQL'
  mysql> SELECT CONCAT('My', NULL, 'QL');
          -> NULL
  mysql> SELECT CONCAT(14.3);
          -> '14.3'
  

• CONCAT_WS(separator,str1,str2,...)

  CONCAT_WS() stands for Concatenate With Separator and is a special form of CONCAT(). The first argument is the separator for the rest of the arguments. The separator is added between the strings to be concatenated. The separator can be a string, as can the rest of the arguments. If the separator is NULL, the result is NULL.

  mysql> SELECT CONCAT_WS(',','First name','Second name','Last Name');
          -> 'First name,Second name,Last Name'
  mysql> SELECT CONCAT_WS(',','First name',NULL,'Last Name');
          -> 'First name,Last Name'
  

  CONCAT_WS() does not skip empty strings. However, it does skip any NULL values after the separator argument.

• CONV(N,from_base,to_base)

  Converts numbers between different number bases. Returns a string representation of the number N, converted from base from_base to base to_base. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If to_base is a negative number, N is regarded as a signed number. Otherwise, N is treated as unsigned. CONV() works with 64-bit precision.

  mysql> SELECT CONV('a',16,2);
          -> '1010'
  mysql> SELECT CONV('6E',18,8);
          -> '172'
  mysql> SELECT CONV(-17,10,-18);
          -> '-H'
  mysql> SELECT CONV(10+'10'+'10'+0xa,10,10);
          -> '40'
  

• ELT(N,str1,str2,str3,...)

  Returns str1 if N = 1, str2 if N = 2, and so on. Returns NULL if N is less than 1 or greater than the number of arguments. ELT() is the complement of FIELD().

  mysql> SELECT ELT(1, 'ej', 'Heja', 'hej', 'foo');
          -> 'ej'
  mysql> SELECT ELT(4, 'ej', 'Heja', 'hej', 'foo');
          -> 'foo'
  

• EXPORT_SET(bits,on,off[,separator[,number_of_bits]])

  Returns a string such that for every bit set in the value bits, you get an on string and for every reset bit, you get an off string. Bits in bits are examined from right to left (from low-order to high-order bits). Strings are added to the result from left to right, separated by the separator string (the default being the comma character ‘,’). The number of bits examined is given by number_of_bits (defaults to 64).

  mysql> SELECT EXPORT_SET(5,'Y','N',',',4);
          -> 'Y,N,Y,N'
  mysql> SELECT EXPORT_SET(6,'1','0',',',10);
          -> '0,1,1,0,0,0,0,0,0,0'
  

• FIELD(str,str1,str2,str3,...)

  Returns the index (position) of str in the str1, str2, str3, ... list. Returns 0 if str is not found.

  If all arguments to FIELD() are strings, all arguments are compared as strings. If all arguments are numbers, they are compared as numbers. Otherwise, the arguments are compared as double.

  If str is NULL, the return value is 0 because NULL fails equality comparison with any value. FIELD() is the complement of ELT().

  mysql> SELECT FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
          -> 2
  mysql> SELECT FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
          -> 0
  

• FIND_IN_SET(str,strlist)

  Returns a value in the range of 1 to N if the string str is in the string list strlist consisting of N substrings. A string list is a string composed of substrings separated by ‘,’ characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic. Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function does not work properly if the first argument contains a comma (‘,’) character.

  mysql> SELECT FIND_IN_SET('b','a,b,c,d');
          -> 2
  

• FORMAT(X,D)

  Formats the number X to a format like '#,###,###.##', rounded to D decimal places, and returns the result as a string. If D is 0, the result has no decimal point or fractional part.

  mysql> SELECT FORMAT(12332.123456, 4);
          -> '12,332.1235'
  mysql> SELECT FORMAT(12332.1,4);
          -> '12,332.1000'
  mysql> SELECT FORMAT(12332.2,0);
          -> '12,332'
  

• HEX(N_or_S)

  If N_or_S is a number, returns a string representation of the hexadecimal value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,16).

  If N_or_S is a string, returns a hexadecimal string representation of N_or_S where each character in N_or_S is converted to two hexadecimal digits.

  mysql> SELECT HEX(255);
          -> 'FF'
  mysql> SELECT 0x616263;
          -> 'abc'
  mysql> SELECT HEX('abc');
          -> 616263
  

• INSERT(str,pos,len,newstr)

  Returns the string str, with the substring beginning at position pos and len characters long replaced by the string newstr. Returns the original string if pos is not within the length of the string. Replaces the rest of the string from position pos is len is not within the length of the rest of the string. Returns NULL if any argument is NULL.

  mysql> SELECT INSERT('Quadratic', 3, 4, 'What');
          -> 'QuWhattic'
  mysql> SELECT INSERT('Quadratic', -1, 4, 'What');
          -> 'Quadratic'
  mysql> SELECT INSERT('Quadratic', 3, 100, 'What');
          -> 'QuWhat'
  

  This function is multi-byte safe.

• INSTR(str,substr)

  Returns the position of the first occurrence of substring substr in string str. This is the same as the two-argument form of LOCATE(), except that the order of the arguments is reversed.

  mysql> SELECT INSTR('foobarbar', 'bar');
          -> 4
  mysql> SELECT INSTR('xbar', 'foobar');
          -> 0
  

  This function is multi-byte safe, and is case sensitive only if at least one argument is a binary string.

• LCASE(str)

  LCASE() is a synonym for LOWER().

• LEFT(str,len)

  Returns the leftmost len characters from the string str.

  mysql> SELECT LEFT('foobarbar', 5);
          -> 'fooba'
  

• LENGTH(str)

  Returns the length of the string str, measured in bytes. A multi-byte character counts as multiple bytes. This means that for a string containing five two-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5.

  mysql> SELECT LENGTH('text');
          -> 4
  

• LOAD_FILE(file_name)

  Reads the file and returns the file contents as a string. To use this function, the file must be located on the server host, you must specify the full pathname to the file, and you must have the FILE privilege. The file must be readable by all and its size less than max_allowed_packet bytes.

  If the file does not exist or cannot be read because one of the preceding conditions is not satisfied, the function returns NULL.

  As of MySQL 5.1.6, the character_set_filesystem system variable controls interpretation of filenames that are given as literal strings.

  mysql> UPDATE t
              SET blob_col=LOAD_FILE('/tmp/picture')
              WHERE id=1;
  

• LOCATE(substr,str), LOCATE(substr,str,pos)

  The first syntax returns the position of the first occurrence of substring substr in string str. The second syntax returns the position of the first occurrence of substring substr in string str, starting at position pos. Returns 0 if substr is not in str.

  mysql> SELECT LOCATE('bar', 'foobarbar');
          -> 4
  mysql> SELECT LOCATE('xbar', 'foobar');
          -> 0
  mysql> SELECT LOCATE('bar', 'foobarbar', 5);
          -> 7
  

  This function is multi-byte safe, and is case-sensitive only if at least one argument is a binary string.

• LOWER(str)

  Returns the string str with all characters changed to lowercase according to the current character set mapping. The default is latin1 (cp1252 West European).

  mysql> SELECT LOWER('QUADRATICALLY');
          -> 'quadratically'
  

  This function is multi-byte safe.

• LPAD(str,len,padstr)

  Returns the string str, left-padded with the string padstr to a length of len characters. If str is longer than len, the return value is shortened to len characters.

  mysql> SELECT LPAD('hi',4,'??');
          -> '??hi'
  mysql> SELECT LPAD('hi',1,'??');
          -> 'h'
  

• LTRIM(str)

  Returns the string str with leading space characters removed.

  mysql> SELECT LTRIM('  barbar');
          -> 'barbar'
  

  This function is multi-byte safe.

• MAKE_SET(bits,str1,str2,...)

  Returns a set value (a string containing substrings separated by ‘,’ characters) consisting of the strings that have the corresponding bit in bits set. str1 corresponds to bit 0, str2 to bit 1, and so on. NULL values in str1, str2, ... are not appended to the result.

  mysql> SELECT MAKE_SET(1,'a','b','c');
          -> 'a'
  mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world');
          -> 'hello,world'
  mysql> SELECT MAKE_SET(1 | 4,'hello','nice',NULL,'world');
          -> 'hello'
  mysql> SELECT MAKE_SET(0,'a','b','c');
          -> ''
  

• MID(str,pos,len)

  MID(str,pos,len) is a synonym for SUBSTRING(str,pos,len).

• OCT(N)

  Returns a string representation of the octal value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,8). Returns NULL if N is NULL.

  mysql> SELECT OCT(12);
          -> '14'
  

• OCTET_LENGTH(str)

  OCTET_LENGTH() is a synonym for LENGTH().

• ORD(str)

  If the leftmost character of the string str is a multi-byte character, returns the code for that character, calculated from the numeric values of its constituent bytes using this formula:

    (1st byte code)
  + (2nd byte code × 256)
  + (3rd byte code × 2562) ...
  

  If the leftmost character is not a multi-byte character, ORD() returns the same value as the ASCII() function.

  mysql> SELECT ORD('2');
          -> 50
  

• POSITION(substr IN str)

  POSITION(substr IN str) is a synonym for LOCATE(substr,str).

• QUOTE(str)

  Quotes a string to produce a result that can be used as a properly escaped data value in an SQL statement. The string is returned enclosed by single quotes and with each instance of single quote (‘'’), backslash (‘\’), ASCII NUL, and Control-Z preceded by a backslash. If the argument is NULL, the return value is the word “NULL” without enclosing single quotes.

  mysql> SELECT QUOTE('Don\'t!');
          -> 'Don\'t!'
  mysql> SELECT QUOTE(NULL);
          -> NULL
  

• REPEAT(str,count)

  Returns a string consisting of the string str repeated count times. If count is less than 1, returns an empty string. Returns NULL if str or count are NULL.

  mysql> SELECT REPEAT('MySQL', 3);
          -> 'MySQLMySQLMySQL'
  

• REPLACE(str,from_str,to_str)

  Returns the string str with all occurrences of the string from_str replaced by the string to_str. REPLACE() performs a case-sensitive match when searching for from_str.

  mysql> SELECT REPLACE('www.mysql.com', 'w', 'Ww');
          -> 'WwWwWw.mysql.com'
  

  This function is multi-byte safe.

• REVERSE(str)

  Returns the string str with the order of the characters reversed.

  mysql> SELECT REVERSE('abc');
          -> 'cba'
  

  This function is multi-byte safe.

• RIGHT(str,len)

  Returns the rightmost len characters from the string str.

  mysql> SELECT RIGHT('foobarbar', 4);
          -> 'rbar'
  

  This function is multi-byte safe.

• RPAD(str,len,padstr)

  Returns the string str, right-padded with the string padstr to a length of len characters. If str is longer than len, the return value is shortened to len characters.

  mysql> SELECT RPAD('hi',5,'?');
          -> 'hi???'
  mysql> SELECT RPAD('hi',1,'?');
          -> 'h'
  

  This function is multi-byte safe.

• RTRIM(str)

  Returns the string str with trailing space characters removed.

  mysql> SELECT RTRIM('barbar   ');
          -> 'barbar'
  

  This function is multi-byte safe.

• SOUNDEX(str)

  Returns a soundex string from str. Two strings that sound almost the same should have identical soundex strings. A standard soundex string is four characters long, but the SOUNDEX() function returns an arbitrarily long string. You can use SUBSTRING() on the result to get a standard soundex string. All non-alphabetic characters in str are ignored. All international alphabetic characters outside the A-Z range are treated as vowels.

  mysql> SELECT SOUNDEX('Hello');
          -> 'H400'
  mysql> SELECT SOUNDEX('Quadratically');
          -> 'Q36324'
  

  Note: This function implements the original Soundex algorithm, not the more popular enhanced version (also described by D. Knuth). The difference is that original version discards vowels first and duplicates second, whereas the enhanced version discards duplicates first and vowels second.

• expr1 SOUNDS LIKE expr2

  This is the same as SOUNDEX(expr1) = SOUNDEX(expr2).

• SPACE(N)

  Returns a string consisting of N space characters.

  mysql> SELECT SPACE(6);
          -> '      '
  

• SUBSTRING(str,pos), SUBSTRING(str FROM pos), SUBSTRING(str,pos,len), SUBSTRING(str FROM pos FOR len)

  The forms without a len argument return a substring from string str starting at position pos. The forms with a len argument return a substring len characters long from string str, starting at position pos. The forms that use FROM are standard SQL syntax. It is also possible to use a negative value for pos. In this case, the beginning of the substring is pos characters from the end of the string, rather than the beginning. A negative value may be used for pos in any of the forms of this function.

  mysql> SELECT SUBSTRING('Quadratically',5);
          -> 'ratically'
  mysql> SELECT SUBSTRING('foobarbar' FROM 4);
          -> 'barbar'
  mysql> SELECT SUBSTRING('Quadratically',5,6);
          -> 'ratica'        
  mysql> SELECT SUBSTRING('Sakila', -3);
          -> 'ila'        
  mysql> SELECT SUBSTRING('Sakila', -5, 3);
          -> 'aki'
  mysql> SELECT SUBSTRING('Sakila' FROM -4 FOR 2);
          -> 'ki'
  

  This function is multi-byte safe.

  If len is less than 1, the result is the empty string.

  SUBSTR() is a synonym for SUBSTRING().

• SUBSTRING_INDEX(str,delim,count)

  Returns the substring from string str before count occurrences of the delimiter delim. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned. SUBSTRING_INDEX() performs a case-sensitive match when searching for delim.

  mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2);
          -> 'www.mysql'
  mysql> SELECT SUBSTRING_INDEX('www.mysql.com', '.', -2);
          -> 'mysql.com'
  

  This function is multi-byte safe.

• TRIM([{BOTH | LEADING | TRAILING} [remstr] FROM] str), TRIM(remstr FROM] str)

  Returns the string str with all remstr prefixes or suffixes removed. If none of the specifiers BOTH, LEADING, or TRAILING is given, BOTH is assumed. remstr is optional and, if not specified, spaces are removed.

  mysql> SELECT TRIM('  bar   ');
          -> 'bar'
  mysql> SELECT TRIM(LEADING 'x' FROM 'xxxbarxxx');
          -> 'barxxx'
  mysql> SELECT TRIM(BOTH 'x' FROM 'xxxbarxxx');
          -> 'bar'
  mysql> SELECT TRIM(TRAILING 'xyz' FROM 'barxxyz');
          -> 'barx'
  

  This function is multi-byte safe.

• UCASE(str)

  UCASE() is a synonym for UPPER().

• UNHEX(str)

  Performs the inverse operation of HEX(str). That is, it interprets each pair of hexadecimal digits in the argument as a number and converts it to the character represented by the number. The resulting characters are returned as a binary string.

  mysql> SELECT UNHEX('4D7953514C');
          -> 'MySQL'
  mysql> SELECT 0x4D7953514C;
          -> 'MySQL'
  mysql> SELECT UNHEX(HEX('string'));
          -> 'string'
  mysql> SELECT HEX(UNHEX('1267'));
          -> '1267'
  

• UPPER(str)

  Returns the string str with all characters changed to uppercase according to the current character set mapping. The default is latin1 (cp1252 West European).

  mysql> SELECT UPPER('Hej');
          -> 'HEJ'
  

  This function is multi-byte safe.