Beta Draft: 2017-02-25
MySQL supports multiple Unicode character sets:
utf8, a UTF-8 encoding of the Unicode character set using one to three bytes per character.
ucs2, the UCS-2 encoding of the Unicode character set using two bytes per character.
utf8mb4, a UTF-8 encoding of the Unicode character set using one to four 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.
Basic Multilingual Plane (BMP) characters.
support BMP and supplementary characters.
This section describes the collations available for Unicode character sets and their differentiating properties. For general information about Unicode, see Section 11.1.9, “Unicode Support”.
Most Unicode character sets have a general collection
_general in the name or by
the absence of a language specifier), a binary collation
_bin in the name), and
several language-specific collations (indicated by language
specifiers). For example, for
utf8_bin are its general and binary
utf8_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
A locale code or 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 11.3 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 that are accent insensitive,
J compare as
compare as equal.
V compare as equal on the base letter
level. In other words,
J is regarded as an
regarded as an accented
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 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.
collations preserve the pre-5.1.24 ordering of the original
collations and permit upgrades for tables created before MySQL
5.1.24. For more information, see
Section 2.10.3, “Checking Whether Tables or Indexes Must Be Rebuilt”, and
Section 2.10.4, “Rebuilding or Repairing Tables or Indexes”.
MySQL implements the
collations according to the Unicode Collation Algorithm (UCA)
collation uses the version-4.0.0 UCA weight keys:
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 later than 4.0.0
include the version in the collation name. Thus,
utf8mb4_unicode_520_ci is based on UCA
5.2.0 weight keys
utf8mb4_0900_ai_ci is based on UCA
9.0.0 weight keys
For collations of
utf8 that include a UCA
version, there is no
utf8mb3 alias; see
Section 18.104.22.168, “The utf8mb3 Character Set (Alias for utf8)”.
MySQL implements language-specific Unicode collations if the ordering based only on UCA does not work well for a language. Language-specific collations are UCA-based, with additional language tailoring rules.
For example, the nonlanguage-specific
utf8mb4_0900_ai_ci and language-specific
Unicode collations each have these characteristics:
The collation is based on Unicode Collation Algorithm (UCA) 9.0.0 and Common Locale Data Repository (CLDR) v30, is accent insensitive, and case insensitive. These characteristics are indicated by
_ciin the collation name. Exception:
utf8mb4_la_0900_ai_ciis not based on CLDR because Classical Latin is not defined in CLDR.
The collation works for all characters in the range [U+0, U+10FFFF].
If the collation is not language specific, it sorts all characters, including supplementary characters, in default order (described following). If the collation is language specific, it sorts characters of the language correctly according to language-specific rules, and characters not in the language in default order.
By default, the collation sorts characters having a code point listed in the DUCET table (Default Unicode Collation Element Table) according to the weight value assigned in the table. The collation sorts characters not having a code point listed in the DUCET table using their implicit weight value, which is constructed according to the UCA.
Exception: For performance reasons, collations based on UCA 9.0.0 sort space characters lower than any other character. The main impact is that space sorts before some whitespace characters that per DUCET it sorts after.
For non-language-specific collations, characters in contraction sequences are treated as separate characters. For language-specific collations, contractions might change character sorting order.
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 is converted by these functions only if
the argument has a collation that uses a recent enough UCA
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 for this is
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
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
To further illustrate, the following equalities hold in both
utf8_unicode_ci (for the effect of this in
comparisons or searches, see
Section 22.214.171.124, “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
which supports the German DIN-1 ordering (also known as
ß = ss
collations if the ordering with
utf8_unicode_ci does not work well for a
language. For example,
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
s, and not to
ss. If this is acceptable for your
application, you should use
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
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
Æ equal to
Œ equal to
utf8_german_ci corresponding to
latin1_german_ci for German dictionary
For all Unicode collations except the binary
_bin) collations, MySQL performs a table
lookup to find a character's collating weight. This weight can
be displayed using the
WEIGHT_STRING() function. (See
Section 13.5, “String Functions”.) If a character is not in
the table (for example, because it is a “new”
character), collating weight determination becomes more
For BMP characters in general collations (
), weight = 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:
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 collation 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
utf16_bin (the binary
utf16) was a binary
comparison “byte by byte” rather than
“character by character.” If that were so, the
order of characters in
differ from the order in
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
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
utf16_bin collation is not
“byte by byte.” It is “by code
point.” When MySQL sees a supplementary-character
utf16, it converts to the
character's code-point value, and then compares. Therefore,
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
ucs2 strings should not
contain surrogates, anyway.