MySQL supports multiple Unicode character sets:

utf8 and ucs2 support Basic Multilingual Plane (BMP) characters. utf8mb4, utf16, and utf32 support BMP and supplementary characters. The utf8mb4, utf16, and utf32 character sets were added in MySQL 5.5.3.

This section describes the collations available for Unicode character sets and their differentiating properties. For general information about Unicode, see Section 10.1.9, “Unicode Support”.

Most Unicode character sets have a general collection (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 utf8, utf8_general_ci and utf8_bin are its general and binary collations, and utf8_danish_ci is one of its language-specific collations.

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.

Danish collations may also be used for Norwegian.

For Classical Latin collations, I and J compare as equal, and U and V compare as equal.

Spanish collations are available for modern and traditional Spanish. For both, 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.

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.

The xxx_general_mysql500_ci collations were added in MySQL 5.5.21. They preserve the pre-5.1.24 ordering of the original xxx_general_ci collations and permit upgrades for tables created before MySQL 5.1.24. For more information, see Section 2.11.3, “Checking Whether Tables or Indexes Must Be Rebuilt”, and Section 2.11.4, “Rebuilding or Repairing Tables or Indexes”.

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. The xxx_unicode_ci 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).

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 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 of this in comparisons or searches, see Section 10.1.8.6, “Examples of the Effect of Collation”):

Ä = A
Ö = O
Ü = U

A difference between the collations is that this is true for 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 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 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. Otherwise, use utf8_unicode_ci because it is more accurate.

For all Unicode collations except the binary (_bin) collations, MySQL performs a table lookup to find a character's collating weight. If a character is not in the table (for example, because it is a “new” character), collating weight determination becomes more complex:

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

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 MySQL 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.