Unicode Character Sets

MySQL 5.6 supports these Unicode character sets:

• ucs2, the UCS-2 encoding of the Unicode character set using 16 bits per character.
• utf16, the UTF-16 encoding for the Unicode character set; like ucs2 but with an extension for supplementary characters.
• utf16le, the UTF-16LE encoding for the Unicode character set; like utf16 but little-endian rather than big-endian.
• utf32, the UTF-32 encoding for the Unicode character set using 32 bits per character.
• utf8, a UTF-8 encoding of the Unicode character set using one to three bytes per character.
• utf8mb4, a UTF-8 encoding of the Unicode character set using one to four bytes per character.

ucs2 and utf8 support Basic Multilingual Plane (BMP) characters. utf8mb4, utf16, utf16le, and utf32 support BMP and supplementary characters. utf16le was added in MariaDB 5.6.1.

You can store text in about 650 languages using these character sets. This section lists the collations available for each Unicode character set and describes their differentiating properties. For general information about the character sets, see , "Unicode Support".

A similar set of collations is available for most Unicode character sets. These are shown in the following list, where xxx represents the character set name. For example, xxx_danish_ci represents the Danish collations, the specific names of which are ucs2_danish_ci, utf16_danish_ci, utf32_danish_ci, utf8_danish_ci, and utf8mb4_danish_ci.

Collation support for utf16le is more limited. The only collations available are utf16le_general_ci and utf16le_bin. These are similar to utf16_general_ci and utf16_bin.

Unicode collation names may also include a version number (for example, xxx_unicode_520_ci) to indicate the Unicode Collation Algorithm version on which the collation is based, as described later in this section. For such collations, there is no utf8mb3 alias to the corresponding utf8 collation. See , "The utf8mb3 "Character Set" (Alias for utf8)".

• xxx_bin
• xxx_croatian_ci
• xxx_czech_ci
• xxx_danish_ci
• xxx_esperanto_ci
• xxx_estonian_ci
• xxx_general_ci (default)
• xxx_german2_ci
• xxx_hungarian_ci
• xxx_icelandic_ci
• xxx_latvian_ci
• xxx_lithuanian_ci
• xxx_persian_ci
• xxx_polish_ci
• xxx_roman_ci
• xxx_romanian_ci
• xxx_sinhala_ci
• xxx_slovak_ci
• xxx_slovenian_ci
• xxx_spanish_ci
• xxx_spanish2_ci
• xxx_swedish_ci
• xxx_turkish_ci
• xxx_unicode_ci
• xxx_vietnamese_ci

MySQL implements the xxx_unicode_ci collations according to the Unicode Collation Algorithm (UCA) described at http://www.unicode.org/reports/tr10/. The collation uses the version-4.0.0 UCA weight keys: http://www.unicode.org/Public/UCA/4.0.0/allkeys-4.0.0.txt. Currently, the xxx_unicode_ci collations have only partial support for the Unicode Collation Algorithm. Some characters are not supported yet. Also, combining marks are not fully supported. This affects primarily Vietnamese, Yoruba, and some smaller languages such as Navajo.

MySQL implements language-specific Unicode collations only if the ordering with xxx_unicode_ci does not work well for a language. Language-specific collations are UCA-based. They are derived from xxx_unicode_ci with additional language tailoring rules.

Collations based on UCA versions later than 4.0.0 include the version in the collation name. Thus, xxx_unicode_520_ci collations are based on UCA 5.2.0 weight keys: http://www.unicode.org/Public/UCA/5.2.0/allkeys.txt.

LOWER() and UPPER() perform case folding according to the collation of their argument. A character that has uppercase and lowercase versions only in a Unicode version more recent than 4.0.0 will be converted by these functions only if the argument has a collation that uses a recent enough UCA version.

For any Unicode character set, operations performed using the xxx_general_ci collation are faster than those for the xxx_unicode_ci collation. For example, comparisons for the utf8_general_ci collation are faster, but slightly less correct, than comparisons for utf8_unicode_ci. The reason for this is that utf8_unicode_ci supports mappings such as expansions; that is, when one character compares as equal to combinations of other characters. For example, in German and some other languages "ß" is equal to "ss". utf8_unicode_ci also supports contractions and ignorable characters. utf8_general_ci is a legacy collation that does not support expansions, contractions, or ignorable characters. It can make only one-to-one comparisons between characters.

To further illustrate, the following equalities hold in both utf8_general_ci and utf8_unicode_ci (for the effect this has in comparisons or when doing searches, see , "Examples of the Effect of Collation"):

Ä = A
Ö = O
Ü = U

A difference between the collations is that this is true for utf8_general_ci:

ß = s

Whereas this is true for utf8_unicode_ci, which supports the German DIN-1 ordering (also known as dictionary order):

ß = ss

MySQL implements language-specific collations for the utf8 character set only if the ordering with utf8_unicode_ci does not work well for a language. For example, utf8_unicode_ci works fine for German dictionary order and French, so there is no need to create special utf8 collations.

utf8_general_ci also is satisfactory for both German and French, except that "ß" is equal to "s", and not to "ss". If this is acceptable for your application, you should use utf8_general_ci because it is faster. If this is not acceptable (for example, if you require German dictionary order), use utf8_unicode_ci because it is more accurate.

If you require German DIN-2 (phone book) ordering, use the utf8_german2_ci collation, which compares the following sets of characters equal:

Ä = Æ = AE
Ö = Œ = OE
Ü = UE
ß = ss

utf8_german2_ci is similar to latin1_german2_ci, but the latter does not compare "Æ" equal to "AE" or "Œ" equal to "OE". There is no utf8_german_ci corresponding to latin1_german_ci for German dictionary order because utf8_general_ci suffices.

xxx_swedish_ci includes Swedish rules. For example, in Swedish, the following relationship holds, which is not something expected by a German or French speaker:

Ü = Y < Ö

The xxx_spanish_ci and xxx_spanish2_ci collations correspond to modern Spanish and traditional Spanish, respectively. In both collations, "ñ" (n-tilde) is a separate letter between "n" and "o". In addition, for traditional Spanish, "ch" is a separate letter between "c" and "d", and "ll" is a separate letter between "l" and "m"

In the xxx_roman_ci collations, I and J compare as equal, and U and V compare as equal.

The xxx_croatian_ci collations are tailored for these Croatian letters: Č, Ć, Dž, Đ, Lj, Nj, Š, Ž.

For all Unicode collations except the "binary" (xxx_bin) collations, MariaDB performs a table lookup to find a character's collating weight. This weight can be displayed using the WEIGHT-STRING() function. (See , "String Functions".) If a character is not in the table (for example, because it is a "new" character), collating weight determination becomes more complex:

• For BMP characters in general collations (xxx_general_ci), weight = code point.
• For BMP characters in UCA collations (for example, xxx_unicode_ci and language-specific collations), the following algorithm applies:

  if (code >= 0x3400 && code <= 0x4DB5)
   base= 0xFB80; /* CJK Ideograph Extension */
  else if (code >= 0x4E00 && code <= 0x9FA5)
   base= 0xFB40; /* CJK Ideograph */
  else
   base= 0xFBC0; /* All other characters */
  aaaa= base + (code >> 15);
  bbbb= (code & 0x7FFF) | 0x8000;
  

  The result is a sequence of two collating elements, aaaa followed by bbbb. For example:

  mysql> SELECT HEX(WEIGHT_STRING(_ucs2 0x04CF COLLATE ucs2_unicode_ci));
  +----------------------------------------------------------+
  | HEX(WEIGHT_STRING(_ucs2 0x04CF COLLATE ucs2_unicode_ci)) |
  +----------------------------------------------------------+
  | FBC084CF |
  +----------------------------------------------------------+
  

  Thus, U+04cf CYRILLIC SMALL LETTER PALOCHKA is, with all UCA 4.0.0 collations, greater than U+04c0 CYRILLIC LETTER PALOCHKA. With UCA 5.2.0 collations, all palochkas sort together.

• For supplementary characters in general collations, the weight is the weight for 0xfffd REPLACEMENT CHARACTER. For supplementary characters in UCA 4.0.0 collations, their collating weight is 0xfffd. That is, to MySQL, all supplementary characters are equal to each other, and greater than almost all BMP characters.

  An example with Deseret characters and COUNT(DISTINCT):

  CREATE TABLE t (s1 VARCHAR(5) CHARACTER SET utf32 COLLATE utf32_unicode_ci);
  INSERT INTO t VALUES (0xfffd); /* REPLACEMENT CHARACTER */
  INSERT INTO t VALUES (0x010412); /* DESERET CAPITAL LETTER BEE */
  INSERT INTO t VALUES (0x010413); /* DESERET CAPITAL LETTER TEE */
  SELECT COUNT(DISTINCT s1) FROM t;
  

  The result is 2 because in the MariaDB xxx_unicode_ci collations, the replacement character has a weight of 0x0dc6, whereas Deseret Bee and Deseret Tee both have a weight of 0xfffd. (Were the utf32_general_ci collation used instead, the result would be 1 because all three characters have a weight of 0xfffd in that collation.)

  An example with cuneiform characters and WEIGHT_STRING():

  /*
  The four characters in the INSERT string are
  00000041 # LATIN CAPITAL LETTER A
  0001218F # CUNEIFORM SIGN KAB
  000121A7 # CUNEIFORM SIGN KISH
  00000042 # LATIN CAPITAL LETTER B
  */
  CREATE TABLE t (s1 CHAR(4) CHARACTER SET utf32 COLLATE utf32_unicode_ci);
  INSERT INTO t VALUES (0x000000410001218f000121a700000042);
  SELECT HEX(WEIGHT_STRING(s1)) FROM t;
  

  The result is:

  0E33 FFFD FFFD 0E4A
  

  0E33 and 0E4A are primary weights as in UCA 4.0.0. FFFD is the weight for KAB and also for KISH.

  The rule that all supplementary characters are equal to each other is nonoptimal but is not expected to cause trouble. These characters are very rare, so it will be very rare that a multi-character string consists entirely of supplementary characters. In Japan, since the supplementary characters are obscure Kanji ideographs, the typical user does not care what order they are in, anyway. If you really want rows sorted by MySQL's rule and secondarily by code point value, it is easy:

  ORDER BY s1 COLLATE utf32_unicode_ci, s1 COLLATE utf32_bin
  

• For supplementary characters based on UCA versions later than 4.0.0 (for example, xxx_unicode_520_ci), supplementary characters do not necessarily all have the same collation weight. Some have explicit weights from the UCA allkeys.txt file. Others have weights calculated from this algorithm:

  aaaa= base + (code >> 15);
  bbbb= (code & 0x7FFF) | 0x8000;
  

The utf16_bin Collation

There is a difference between "ordering by the character's code value" and "ordering by the character's binary representation," a difference that appears only with utf16_bin, because of surrogates.

Suppose that utf16_bin (the binary collation for utf16) was a binary comparison "byte by byte" rather than "character by character." If that were so, the order of characters in utf16_bin would differ from the order in utf8_bin. For example, the following chart shows two rare characters. The first character is in the range E000-FFFF, so it is greater than a surrogate but less than a supplementary. The second character is a supplementary.

Code point Character utf8 utf16
---------- --------- ---- -----
0FF9D HALFWIDTH KATAKANA LETTER N EF BE 9D FF 9D
10384 UGARITIC LETTER DELTA F0 90 8E 84 D8 00 DF 84

The two characters in the chart are in order by code point value because 0xff9d < 0x10384. And they are in order by utf8 value because 0xef < 0xf0. But they are not in order by utf16 value, if we use byte-by-byte comparison, because 0xff > 0xd8.

So MySQL's utf16_bin collation is not "byte by byte." It is "by code point." When MariaDB sees a supplementary-character encoding in utf16, it converts to the character's code-point value, and then compares. Therefore, utf8_bin and utf16_bin are the same ordering. This is consistent with the SQL:2008 standard requirement for a UCS_BASIC collation: "UCS_BASIC is a collation in which the ordering is determined entirely by the Unicode scalar values of the characters in the strings being sorted. It is applicable to the UCS character repertoire. Since every character repertoire is a subset of the UCS repertoire, the UCS_BASIC collation is potentially applicable to every character set. NOTE 11: The Unicode scalar value of a character is its code point treated as an unsigned integer."

If the character set is ucs2, comparison is byte-by-byte, but ucs2 strings should not contain surrogates, anyway.

For additional information about Unicode collations in MySQL, see Collation-Charts.Org (utf8).

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