10.10.1 Unicode Character Sets
This section describes the collations available for Unicode character sets and their differentiating properties. For general information about Unicode, see Section 10.9, “Unicode Support”.
MySQL supports multiple Unicode character sets:
utf8mb4: A UTF-8 encoding of the Unicode character set using one to four bytes per character.
utf8mb3: A UTF-8 encoding of the Unicode character set using one to three bytes per character.
utf8: An alias for
ucs2: The UCS-2 encoding of the Unicode character set using two bytes per character.
utf16: The UTF-16 encoding for the Unicode character set using two or four bytes per character. Like
ucs2but with an extension for supplementary characters.
utf16le: The UTF-16LE encoding for the Unicode character set. Like
utf16but little-endian rather than big-endian.
utf32: The UTF-32 encoding for the Unicode character set using four bytes per character.
utf32 support Basic Multilingual Plane (BMP) characters and supplementary characters that lie outside the BMP.
ucs2 support only BMP characters.
Most Unicode character sets have a general collation (indicated by
_general in the name or by the absence of a language specifier), a binary collation (indicated by
_bin in the name), and several language-specific collations (indicated by language specifiers). For example, for
utf8mb4_bin are its general and binary collations, and
utf8mb4_danish_ci is one of its language-specific collations.
Collation support for
utf16le is limited. The only collations available are
utf16le_bin. These are similar to
MySQL implements the
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 . The
collations have only partial support for the Unicode Collation Algorithm. Some characters are not supported, and combining marks are not fully supported. This affects primarily Vietnamese, Yoruba, and some smaller languages such as Navajo. A combined character is considered different from the same character written with a single unicode character in string comparisons, and the two characters are considered to have a different length (for example, as returned by the
CHAR_LENGTH() function or in result set metadata).
Unicode collations based on UCA versions higher than 4.0.0 include the version in the collation name. Thus,
utf8_unicode_520_ci is based on UCA 5.2.0 weight keys (http://www.unicode.org/Public/UCA/5.2.0/allkeys.txt ).
UPPER() functions perform case folding according to the collation of their argument. A character that has uppercase and lowercase versions only in a Unicode version higher than 4.0.0 is converted by these functions only if the argument collation uses a high enough UCA version.
MySQL implements language-specific Unicode collations if the ordering based only on the Unicode Collation Algorithm (UCA) does not work well for a language. Language-specific collations are UCA-based, with additional language tailoring rules. Examples of such rules appear later in this section. For questions about particular language orderings, unicode.org provides Common Locale Data Repository (CLDR) collation charts at http://www.unicode.org/cldr/charts/30/collation/index.html .
A language name shown in the following table indicates a language-specific collation. Unicode character sets may include collations for one or more of these languages.
Table 10.3 Unicode Collation Language Specifiers
|German phone book order||
Croatian collations are tailored for these Croatian letters:
Danish collations may also be used for Norwegian.
For Classical Latin collations,
J compare as equal, and
V compare as equal.
Spanish collations are available for modern and traditional Spanish. For both,
ñ (n-tilde) is a separate letter between
o. In addition, for traditional Spanish,
ch is a separate letter between
ll is a separate letter between
Traditional Spanish collations may also be used for Asturian and Galician.
Swedish collations include Swedish rules. For example, in Swedish, the following relationship holds, which is not something expected by a German or French speaker:
Ü = Y < Ö
For any Unicode character set, operations performed using the
collation are faster than those for the
collation. For example, comparisons for the
utf8_general_ci collation are faster, but slightly less correct, than comparisons for
utf8_unicode_ci. The reason 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,
ß is equal to
ss in German and some other languages.
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_unicode_ci (for the effect of this in comparisons or searches, see Section 10.8.6, “Examples of the Effect of Collation”):
Ä = A Ö = O Ü = U
A difference between the collations is that this is true for
ß = s
Whereas this is true for
utf8_unicode_ci, which supports the German DIN-1 ordering (also known as dictionary order):
ß = ss
utf8 language-specific collations 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_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
Œ equal to
OE. There is no
utf8_german_ci corresponding to
latin1_german_ci for German dictionary order because
A character's collating weight is determined as follows:
For all Unicode collations except the
_bin(binary) collations, MySQL performs a table lookup to find a character's collating weight.
_bincollations, the weight is based on the code point, possibly with leading zero bytes added.
Collating weights can be displayed using the
WEIGHT_STRING() function. (See Section 12.7, “String Functions and Operators”.) If a collation uses a weight lookup table, but 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 (
), the weight is the code point.
For BMP characters in UCA collations (for example,
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,
bbbb. For example:
mysql> SELECT HEX(WEIGHT_STRING(_ucs2 0x04CF COLLATE ucs2_unicode_ci)); +----------------------------------------------------------+ | HEX(WEIGHT_STRING(_ucs2 0x04CF COLLATE ucs2_unicode_ci)) | +----------------------------------------------------------+ | FBC084CF | +----------------------------------------------------------+
U+04cf CYRILLIC SMALL LETTER PALOCHKAis, 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
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 MySQL
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_cicollation used instead, the result is 1 because all three characters have a weight of
0xfffdin that collation.)
An example with cuneiform characters and
/* 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
0E4Aare primary weights as in UCA 4.0.0 .
FFFDis 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 is 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 the MySQL 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 higher than 4.0.0 (for example,
), supplementary characters do not necessarily all have the same collating weight. Some have explicit weights from the UCA
allkeys.txtfile. Others have weights calculated from this algorithm:
aaaa= base + (code >> 15); bbbb= (code & 0x7FFF) | 0x8000;
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.
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
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
0x10384. And they are in order by
utf8 value because
0xf0. But they are not in order by
utf16 value, if we use byte-by-byte comparison, because
utf16_bin collation is not “byte by byte.” It is “by code point.” When MySQL sees a supplementary-character encoding in
utf16, it converts to the character's code-point value, and then compares. Therefore,
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.