14.8 String Functions and Operators

14.8.1 String Comparison Functions and Operators

14.8.2 Regular Expressions

14.8.3 Character Set and Collation of Function Results

Table 14.12 String Functions and Operators

Name	Description
ASCII()	Return numeric value of left-most character
BIN()	Return a string containing binary representation of a number
BIT_LENGTH()	Return length of argument in bits
CHAR()	Return the character for each integer passed
CHAR_LENGTH()	Return number of characters in argument
CHARACTER_LENGTH()	Synonym for CHAR_LENGTH()
CONCAT()	Return concatenated string
CONCAT_WS()	Return concatenate with separator
ELT()	Return string at index number
EXPORT_SET()	Return a string such that for every bit set in the value bits, you get an on string and for every unset bit, you get an off string
FIELD()	Index (position) of first argument in subsequent arguments
FIND_IN_SET()	Index (position) of first argument within second argument
FORMAT()	Return a number formatted to specified number of decimal places
FROM_BASE64()	Decode base64 encoded string and return result
HEX()	Hexadecimal representation of decimal or string value
INSERT()	Insert substring at specified position up to specified number of characters
INSTR()	Return the index of the first occurrence of substring
LCASE()	Synonym for LOWER()
LEFT()	Return the leftmost number of characters as specified
LENGTH()	Return the length of a string in bytes
LIKE	Simple pattern matching
LOAD_FILE()	Load the named file
LOCATE()	Return the position of the first occurrence of substring
LOWER()	Return the argument in lowercase
LPAD()	Return the string argument, left-padded with the specified string
LTRIM()	Remove leading spaces
MAKE_SET()	Return a set of comma-separated strings that have the corresponding bit in bits set
MATCH()	Perform full-text search
MID()	Return a substring starting from the specified position
NOT LIKE	Negation of simple pattern matching
NOT REGEXP	Negation of REGEXP
OCT()	Return a string containing octal representation of a number
OCTET_LENGTH()	Synonym for LENGTH()
ORD()	Return character code for leftmost character of the argument
POSITION()	Synonym for LOCATE()
QUOTE()	Escape the argument for use in an SQL statement
REGEXP	Whether string matches regular expression
REGEXP_INSTR()	Starting index of substring matching regular expression
REGEXP_LIKE()	Whether string matches regular expression
REGEXP_REPLACE()	Replace substrings matching regular expression
REGEXP_SUBSTR()	Return substring matching regular expression
REPEAT()	Repeat a string the specified number of times
REPLACE()	Replace occurrences of a specified string
REVERSE()	Reverse the characters in a string
RIGHT()	Return the specified rightmost number of characters
RLIKE	Whether string matches regular expression
RPAD()	Append string the specified number of times
RTRIM()	Remove trailing spaces
SOUNDEX()	Return a soundex string
SOUNDS LIKE	Compare sounds
SPACE()	Return a string of the specified number of spaces
STRCMP()	Compare two strings
SUBSTR()	Return the substring as specified
SUBSTRING()	Return the substring as specified
SUBSTRING_INDEX()	Return a substring from a string before the specified number of occurrences of the delimiter
TO_BASE64()	Return the argument converted to a base-64 string
TRIM()	Remove leading and trailing spaces
UCASE()	Synonym for UPPER()
UNHEX()	Return a string containing hex representation of a number
UPPER()	Convert to uppercase
WEIGHT_STRING()	Return the weight string for a string

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 7.1.1, “Configuring the Server”.

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

For functions that take length arguments, noninteger arguments are rounded to the nearest integer.

• 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 8-bit characters.

  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. Returns NULL if str is NULL.

  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');
  +--------------------------------------------------+
  | CHAR(77,121,83,81,'76')                          |
  +--------------------------------------------------+
  | 0x4D7953514C                                     |
  +--------------------------------------------------+
  1 row in set (0.00 sec)
  
  mysql> SELECT CHAR(77,77.3,'77.3');
  +--------------------------------------------+
  | CHAR(77,77.3,'77.3')                       |
  +--------------------------------------------+
  | 0x4D4D4D                                   |
  +--------------------------------------------+
  1 row in set (0.00 sec)

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

  mysql> SELECT CHAR(77,121,83,81,'76' USING utf8mb4);
  +---------------------------------------+
  | CHAR(77,121,83,81,'76' USING utf8mb4) |
  +---------------------------------------+
  | MySQL                                 |
  +---------------------------------------+
  1 row in set (0.00 sec)
  
  mysql> SELECT CHAR(77,77.3,'77.3' USING utf8mb4);
  +------------------------------------+
  | CHAR(77,77.3,'77.3' USING utf8mb4) |
  +------------------------------------+
  | MMM                                |
  +------------------------------------+
  1 row in set, 1 warning (0.00 sec)
  
  mysql> SHOW WARNINGS;
  +---------+------+-------------------------------------------+
  | Level   | Code | Message                                   |
  +---------+------+-------------------------------------------+
  | Warning | 1292 | Truncated incorrect INTEGER value: '77.3' |
  +---------+------+-------------------------------------------+
  1 row in set (0.00 sec)

  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.

  If CHAR() is invoked from within the mysql client, binary strings display using hexadecimal notation, depending on the value of the --binary-as-hex. For more information about that option, see Section 6.5.1, “mysql — The MySQL Command-Line Client”.

  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           |
  +----------------+----------------+
  1 row in set (0.00 sec)
  
  mysql> SELECT HEX(CHAR(1,0,0)), HEX(CHAR(256*256));
  +------------------+--------------------+
  | HEX(CHAR(1,0,0)) | HEX(CHAR(256*256)) |
  +------------------+--------------------+
  | 010000           | 010000             |
  +------------------+--------------------+
  1 row in set (0.00 sec)

• CHAR_LENGTH(str)

  Returns the length of the string str, measured in code points. A multibyte character counts as a single code point. This means that, for a string containing two 3-byte characters, LENGTH() returns 6, whereas CHAR_LENGTH() returns 2, as shown here:

  mysql> SET @dolphin:='海豚';
  Query OK, 0 rows affected (0.01 sec)
  
  mysql> SELECT LENGTH(@dolphin), CHAR_LENGTH(@dolphin);
  +------------------+-----------------------+
  | LENGTH(@dolphin) | CHAR_LENGTH(@dolphin) |
  +------------------+-----------------------+
  |                6 |                     2 |
  +------------------+-----------------------+
  1 row in set (0.00 sec)

  CHAR_LENGTH() returns NULL if str is NULL.

• 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 nonbinary strings, the result is a nonbinary string. If the arguments include any binary strings, the result is a binary string. A numeric argument is converted to its equivalent nonbinary string form.

  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'

  For quoted strings, concatenation can be performed by placing the strings next to each other:

  mysql> SELECT 'My' 'S' 'QL';
          -> 'MySQL'

  If CONCAT() is invoked from within the mysql client, binary string results display using hexadecimal notation, depending on the value of the --binary-as-hex. For more information about that option, see Section 6.5.1, “mysql — The MySQL Command-Line Client”.

• 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.

• ELT(N,str1,str2,str3,...)

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

  mysql> SELECT ELT(1, 'Aa', 'Bb', 'Cc', 'Dd');
          -> 'Aa'
  mysql> SELECT ELT(4, 'Aa', 'Bb', 'Cc', 'Dd');
          -> 'Dd'

• 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 bit not set in the value, 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, which has a default of 64 if not specified. number_of_bits is silently clipped to 64 if larger than 64. It is treated as an unsigned integer, so a value of −1 is effectively the same as 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('Bb', 'Aa', 'Bb', 'Cc', 'Dd', 'Ff');
          -> 2
  mysql> SELECT FIELD('Gg', 'Aa', 'Bb', 'Cc', 'Dd', 'Ff');
          -> 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[,locale])

  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. If X or D is NULL, the function returns NULL.

  The optional third parameter enables a locale to be specified to be used for the result number's decimal point, thousands separator, and grouping between separators. Permissible locale values are the same as the legal values for the lc_time_names system variable (see Section 12.16, “MySQL Server Locale Support”). If the locale is NULL or not specified, the default locale is 'en_US'.

  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'
  mysql> SELECT FORMAT(12332.2,2,'de_DE');
          -> '12.332,20'

• FROM_BASE64(str)

  Takes a string encoded with the base-64 encoded rules used by TO_BASE64() and returns the decoded result as a binary string. The result is NULL if the argument is NULL or not a valid base-64 string. See the description of TO_BASE64() for details about the encoding and decoding rules.

  mysql> SELECT TO_BASE64('abc'), FROM_BASE64(TO_BASE64('abc'));
          -> 'JWJj', 'abc'

  If FROM_BASE64() is invoked from within the mysql client, binary strings display using hexadecimal notation. You can disable this behavior by setting the value of the --binary-as-hex to 0 when starting the mysql client. For more information about that option, see Section 6.5.1, “mysql — The MySQL Command-Line Client”.

• HEX(str), HEX(N)

  For a string argument str, HEX() returns a hexadecimal string representation of str where each byte of each character in str is converted to two hexadecimal digits. (Multibyte characters therefore become more than two digits.) The inverse of this operation is performed by the UNHEX() function.

  For a numeric argument N, HEX() returns a hexadecimal string representation of the value of N treated as a longlong (BIGINT) number. This is equivalent to CONV(N,10,16). The inverse of this operation is performed by CONV(HEX(N),16,10).

  For a NULL argument, this function returns NULL.

  mysql> SELECT X'616263', HEX('abc'), UNHEX(HEX('abc'));
          -> 'abc', 616263, 'abc'
  mysql> SELECT HEX(255), CONV(HEX(255),16,10);
          -> 'FF', 255

• 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 if 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 multibyte 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 multibyte safe, and is case-sensitive only if at least one argument is a binary string. If either argument is NULL, this functions returns NULL.

• LCASE(str)

  LCASE() is a synonym for LOWER().

  LCASE() used in a view is rewritten as LOWER() when storing the view's definition. (Bug #12844279)

• LEFT(str,len)

  Returns the leftmost len characters from the string str, or NULL if any argument is NULL.

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

  This function is multibyte safe.

• LENGTH(str)

  Returns the length of the string str, measured in bytes. A multibyte character counts as multiple bytes. This means that for a string containing five 2-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5. Returns NULL if str is NULL.

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

  Note

  The Length() OpenGIS spatial function is named ST_Length() in MySQL.

• 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 path name to the file, and you must have the FILE privilege. The file must be readable by the server and its size less than max_allowed_packet bytes. If the secure_file_priv system variable is set to a nonempty directory name, the file to be loaded must be located in that directory.

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

  The character_set_filesystem system variable controls interpretation of file names 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. Returns NULL if any argument is NULL.

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

  This function is multibyte 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, or NULL if str is NULL. The default character set is utf8mb4.

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

  LOWER() (and UPPER()) are ineffective when applied to binary strings (BINARY, VARBINARY, BLOB). To perform lettercase conversion of a binary string, first convert it to a nonbinary string using a character set appropriate for the data stored in the string:

  mysql> SET @str = BINARY 'New York';
  mysql> SELECT LOWER(@str), LOWER(CONVERT(@str USING utf8mb4));
  +-------------+------------------------------------+
  | LOWER(@str) | LOWER(CONVERT(@str USING utf8mb4)) |
  +-------------+------------------------------------+
  | New York    | new york                           |
  +-------------+------------------------------------+

  For collations of Unicode character sets, LOWER() and UPPER() work according to the Unicode Collation Algorithm (UCA) version in the collation name, if there is one, and UCA 4.0.0 if no version is specified. For example, utf8mb4_0900_ai_ci and utf8mb3_unicode_520_ci work according to UCA 9.0.0 and 5.2.0, respectively, whereas utf8mb3_unicode_ci works according to UCA 4.0.0. See Section 12.10.1, “Unicode Character Sets”.

  This function is multibyte safe.

  LCASE() used within views is rewritten as LOWER().

• 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'

  Returns NULL if any of its arguments are NULL.

• LTRIM(str)

  Returns the string str with leading space characters removed. Returns NULL if str is NULL.

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

  This function is multibyte 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), MID(str FROM pos), MID(str,pos,len), MID(str FROM pos FOR 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 multibyte 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 * 256^2) ...

  If the leftmost character is not a multibyte character, ORD() returns the same value as the ASCII() function. The function returns NULL if str is NULL.

  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 quotation marks and with each instance of backslash (\), single quote ('), ASCII NUL, and Control+Z preceded by a backslash. If the argument is NULL, the return value is the word “NULL” without enclosing single quotation marks.

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

  For comparison, see the quoting rules for literal strings and within the C API in Section 11.1.1, “String Literals”, and mysql_real_escape_string_quote().

• 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 is 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 multibyte safe. It returns NULL if any of its arguments are NULL.

• REVERSE(str)

  Returns the string str with the order of the characters reversed, or NULL if str is NULL.

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

  This function is multibyte safe.

• RIGHT(str,len)

  Returns the rightmost len characters from the string str, or NULL if any argument is NULL.

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

  This function is multibyte 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. If str, padstr, or len is NULL, the function returns NULL.

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

  This function is multibyte safe.

• RTRIM(str)

  Returns the string str with trailing space characters removed.

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

  This function is multibyte safe, and returns NULL if str is NULL.

• SOUNDEX(str)

  Returns a soundex string from str, or NULL if str is NULL. 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 nonalphabetic characters in str are ignored. All international alphabetic characters outside the A-Z range are treated as vowels.

  Important

  When using SOUNDEX(), you should be aware of the following limitations:

  • This function, as currently implemented, is intended to work well with strings that are in the English language only. Strings in other languages may not produce reliable results.

  • This function is not guaranteed to provide consistent results with strings that use multibyte character sets, including utf-8. See Bug #22638 for more information.

  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, or NULL if N is NULL.

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

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

  SUBSTR() is a synonym for SUBSTRING().

• 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. A value of 0 for pos returns an empty string.

  For all forms of SUBSTRING(), the position of the first character in the string from which the substring is to be extracted is reckoned as 1.

  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 multibyte safe. It returns NULL if any of its arguments are NULL.

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

• 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 multibyte safe.

  SUBSTRING_INDEX() returns NULL if any of its arguments are NULL.

• TO_BASE64(str)

  Converts the string argument to base-64 encoded form and returns the result as a character string with the connection character set and collation. If the argument is not a string, it is converted to a string before conversion takes place. The result is NULL if the argument is NULL. Base-64 encoded strings can be decoded using the FROM_BASE64() function.

  mysql> SELECT TO_BASE64('abc'), FROM_BASE64(TO_BASE64('abc'));
          -> 'JWJj', 'abc'

  Different base-64 encoding schemes exist. These are the encoding and decoding rules used by TO_BASE64() and FROM_BASE64():

  • The encoding for alphabet value 62 is '+'.

  • The encoding for alphabet value 63 is '/'.

  • Encoded output consists of groups of 4 printable characters. Each 3 bytes of the input data are encoded using 4 characters. If the last group is incomplete, it is padded with '=' characters to a length of 4.

  • A newline is added after each 76 characters of encoded output to divide long output into multiple lines.

  • Decoding recognizes and ignores newline, carriage return, tab, and space.

• 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 multibyte safe. It returns NULL if any of its arguments are NULL.

• UCASE(str)

  UCASE() is a synonym for UPPER().

  UCASE() used within views is rewritten as UPPER().

• UNHEX(str)

  For a string argument str, UNHEX(str) interprets each pair of characters in the argument as a hexadecimal number and converts it to the byte represented by the number. The return value is a binary string.

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

  The characters in the argument string must be legal hexadecimal digits: '0' .. '9', 'A' .. 'F', 'a' .. 'f'. If the argument contains any nonhexadecimal digits, or is itself NULL, the result is NULL:

  mysql> SELECT UNHEX('GG');
  +-------------+
  | UNHEX('GG') |
  +-------------+
  | NULL        |
  +-------------+
  
  mysql> SELECT UNHEX(NULL);
  +-------------+
  | UNHEX(NULL) |
  +-------------+
  | NULL        |
  +-------------+

  A NULL result can also occur if the argument to UNHEX() is a BINARY column, because values are padded with 0x00 bytes when stored but those bytes are not stripped on retrieval. For example, '41' is stored into a CHAR(3) column as '41 ' and retrieved as '41' (with the trailing pad space stripped), so UNHEX() for the column value returns X'41'. By contrast, '41' is stored into a BINARY(3) column as '41\0' and retrieved as '41\0' (with the trailing pad 0x00 byte not stripped). '\0' is not a legal hexadecimal digit, so UNHEX() for the column value returns NULL.

  For a numeric argument N, the inverse of HEX(N) is not performed by UNHEX(). Use CONV(HEX(N),16,10) instead. See the description of HEX().

  If UNHEX() is invoked from within the mysql client, binary strings display using hexadecimal notation, depending on the value of the --binary-as-hex. For more information about that option, see Section 6.5.1, “mysql — The MySQL Command-Line Client”.

• UPPER(str)

  Returns the string str with all characters changed to uppercase according to the current character set mapping, or NULL if str is NULL. The default character set is utf8mb4.

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

  See the description of LOWER() for information that also applies to UPPER(). This included information about how to perform lettercase conversion of binary strings (BINARY, VARBINARY, BLOB) for which these functions are ineffective, and information about case folding for Unicode character sets.

  This function is multibyte safe.

  UCASE() used within views is rewritten as UPPER().

• WEIGHT_STRING(str [AS {CHAR|BINARY}(N)] [flags])

  This function returns the weight string for the input string. The return value is a binary string that represents the comparison and sorting value of the string, or NULL if the argument is NULL. It has these properties:

  • If WEIGHT_STRING(str1) = WEIGHT_STRING(str2), then str1 = str2 (str1 and str2 are considered equal)

  • If WEIGHT_STRING(str1) < WEIGHT_STRING(str2), then str1 < str2 (str1 sorts before str2)

  WEIGHT_STRING() is a debugging function intended for internal use. Its behavior can change without notice between MySQL versions. It can be used for testing and debugging of collations, especially if you are adding a new collation. See Section 12.14, “Adding a Collation to a Character Set”.

  This list briefly summarizes the arguments. More details are given in the discussion following the list.

  • str: The input string expression.

  • AS clause: Optional; cast the input string to a given type and length.

  • flags: Optional; unused.

  The input string, str, is a string expression. If the input is a nonbinary (character) string such as a CHAR, VARCHAR, or TEXT value, the return value contains the collation weights for the string. If the input is a binary (byte) string such as a BINARY, VARBINARY, or BLOB value, the return value is the same as the input (the weight for each byte in a binary string is the byte value). If the input is NULL, WEIGHT_STRING() returns NULL.

  Examples:

  mysql> SET @s = _utf8mb4 'AB' COLLATE utf8mb4_0900_ai_ci;
  mysql> SELECT @s, HEX(@s), HEX(WEIGHT_STRING(@s));
  +------+---------+------------------------+
  | @s   | HEX(@s) | HEX(WEIGHT_STRING(@s)) |
  +------+---------+------------------------+
  | AB   | 4142    | 1C471C60               |
  +------+---------+------------------------+

  mysql> SET @s = _utf8mb4 'ab' COLLATE utf8mb4_0900_ai_ci;
  mysql> SELECT @s, HEX(@s), HEX(WEIGHT_STRING(@s));
  +------+---------+------------------------+
  | @s   | HEX(@s) | HEX(WEIGHT_STRING(@s)) |
  +------+---------+------------------------+
  | ab   | 6162    | 1C471C60               |
  +------+---------+------------------------+

  mysql> SET @s = CAST('AB' AS BINARY);
  mysql> SELECT @s, HEX(@s), HEX(WEIGHT_STRING(@s));
  +------+---------+------------------------+
  | @s   | HEX(@s) | HEX(WEIGHT_STRING(@s)) |
  +------+---------+------------------------+
  | AB   | 4142    | 4142                   |
  +------+---------+------------------------+

  mysql> SET @s = CAST('ab' AS BINARY);
  mysql> SELECT @s, HEX(@s), HEX(WEIGHT_STRING(@s));
  +------+---------+------------------------+
  | @s   | HEX(@s) | HEX(WEIGHT_STRING(@s)) |
  +------+---------+------------------------+
  | ab   | 6162    | 6162                   |
  +------+---------+------------------------+

  The preceding examples use HEX() to display the WEIGHT_STRING() result. Because the result is a binary value, HEX() can be especially useful when the result contains nonprinting values, to display it in printable form:

  mysql> SET @s = CONVERT(X'C39F' USING utf8mb4) COLLATE utf8mb4_czech_ci;
  mysql> SELECT HEX(WEIGHT_STRING(@s));
  +------------------------+
  | HEX(WEIGHT_STRING(@s)) |
  +------------------------+
  | 0FEA0FEA               |
  +------------------------+

  For non-NULL return values, the data type of the value is VARBINARY if its length is within the maximum length for VARBINARY, otherwise the data type is BLOB.

  The AS clause may be given to cast the input string to a nonbinary or binary string and to force it to a given length:

  • AS CHAR(N) casts the string to a nonbinary string and pads it on the right with spaces to a length of N characters. N must be at least 1. If N is less than the length of the input string, the string is truncated to N characters. No warning occurs for truncation.

  • AS BINARY(N) is similar but casts the string to a binary string, N is measured in bytes (not characters), and padding uses 0x00 bytes (not spaces).

  mysql> SET NAMES 'latin1';
  mysql> SELECT HEX(WEIGHT_STRING('ab' AS CHAR(4)));
  +-------------------------------------+
  | HEX(WEIGHT_STRING('ab' AS CHAR(4))) |
  +-------------------------------------+
  | 41422020                            |
  +-------------------------------------+
  mysql> SET NAMES 'utf8mb4';
  mysql> SELECT HEX(WEIGHT_STRING('ab' AS CHAR(4)));
  +-------------------------------------+
  | HEX(WEIGHT_STRING('ab' AS CHAR(4))) |
  +-------------------------------------+
  | 1C471C60                            |
  +-------------------------------------+

  mysql> SELECT HEX(WEIGHT_STRING('ab' AS BINARY(4)));
  +---------------------------------------+
  | HEX(WEIGHT_STRING('ab' AS BINARY(4))) |
  +---------------------------------------+
  | 61620000                              |
  +---------------------------------------+

  The flags clause currently is unused.

  If WEIGHT_STRING() is invoked from within the mysql client, binary strings display using hexadecimal notation, depending on the value of the --binary-as-hex. For more information about that option, see Section 6.5.1, “mysql — The MySQL Command-Line Client”.