Chapter聽10.聽Character Set Support - MySQL 4.1参考手册英文版

MySQL Server has a server character set and a server collation. These can be set at server startup and changed at runtime.

Initially, the server character set and collation depend on the options that you use when you start mysqld. You can use --character-set-server for the character set. Along with it, you can add --collation-server for the collation. If you don't specify a character set, that is the same as saying --character-set-server=latin1. If you specify only a character set (for example, latin1) but not a collation, that is the same as saying --character-set-server=latin1 --collation-server=latin1_swedish_ci because latin1_swedish_ci is the default collation for latin1. Therefore, the following three commands all have the same effect:

shell> mysqld
shell> mysqld --character-set-server=latin1
shell> mysqld --character-set-server=latin1 \
           --collation-server=latin1_swedish_ci

One way to change the settings is by recompiling. If you want to change the default server character set and collation when building from sources, use: --with-charset and --with-collation as arguments for configure. For example:

shell> ./configure --with-charset=latin1

Or:

shell> ./configure --with-charset=latin1 \
           --with-collation=latin1_german1_ci

Both mysqld and configure verify that the character set/collation combination is valid. If not, each program displays an error message and terminates.

The current server character set and collation can be determined from the values of the character_set_server and collation_server system variables. These variables can be changed at runtime.

Every database has a database character set and a database collation. The CREATE DATABASE and ALTER DATABASE statements have optional clauses for specifying the database character set and collation:

CREATE DATABASE db_name
    [[DEFAULT] CHARACTER SET charset_name]
    [[DEFAULT] COLLATE collation_name]

ALTER DATABASE db_name
    [[DEFAULT] CHARACTER SET charset_name]
    [[DEFAULT] COLLATE collation_name]

All database options are stored in a text file named db.opt that can be found in the database directory.

The CHARACTER SET and COLLATE clauses make it possible to create databases with different character sets and collations on the same MySQL server.

Example:

CREATE DATABASE db_name CHARACTER SET latin1 COLLATE latin1_swedish_ci;

MySQL chooses the database character set and database collation in the following manner:

The database character set and collation are used as default values if the table character set and collation are not specified in CREATE TABLE statements. They have no other purpose.

The character set and collation for the default database can be determined from the values of the character_set_database and collation_database system variables. The server sets these variables whenever the default database changes. If there is no default database, the variables have the same value as the corresponding server-level system variables, character_set_server and collation_server.

Every table has a table character set and a table collation. The CREATE TABLE and ALTER TABLE statements have optional clauses for specifying the table character set and collation:

CREATE TABLE tbl_name (column_list)
    [[DEFAULT] CHARACTER SET charset_name] [COLLATE collation_name]]

ALTER TABLE tbl_name
    [[DEFAULT] CHARACTER SET charset_name] [COLLATE collation_name]

Example:

CREATE TABLE t1 ( ... ) CHARACTER SET latin1 COLLATE latin1_danish_ci;

MySQL chooses the table character set and collation in the following manner:

The table character set and collation are used as default values if the column character set and collation are not specified in individual column definitions. The table character set and collation are MySQL extensions; there are no such things in standard SQL.

Every 鈥�character鈥� column (that is, a column of type CHAR, VARCHAR, or TEXT) has a column character set and a column collation. Column definition syntax has optional clauses for specifying the column character set and collation:

col_name {CHAR | VARCHAR | TEXT} (col_length)
    [CHARACTER SET charset_name] [COLLATE collation_name]

Example:

CREATE TABLE Table1
(
    column1 VARCHAR(5) CHARACTER SET latin1 COLLATE latin1_german1_ci
);

MySQL chooses the column character set and collation in the following manner:

The CHARACTER SET and COLLATE clauses are standard SQL.

Every character string literal has a character set and a collation.

A character string literal may have an optional character set introducer and COLLATE clause:

[_charset_name]'string' [COLLATE collation_name]

Examples:

SELECT 'string';
SELECT _latin1'string';
SELECT _latin1'string' COLLATE latin1_danish_ci;

For the simple statement SELECT 'string', the string has the character set and collation defined by the character_set_connection and collation_connection system variables.

The _charset_name expression is formally called an introducer. It tells the parser, 鈥�the string that is about to follow uses character set X.鈥� Because this has confused people in the past, we emphasize that an introducer does not cause any conversion; it is strictly a signal that does not change the string's value. An introducer is also legal before standard hex literal and numeric hex literal notation (x'literal' and 0xnnnn)>.

Examples:

SELECT _latin1 x'AABBCC';
SELECT _latin1 0xAABBCC;

MySQL determines a literal's character set and collation in the following manner:

Examples:

Character set introducers and the COLLATE clause are implemented according to standard SQL specifications.

An introducer indicates the character set for the following string, but does not change now how the parser performs escape processing within the string. Escapes are always interpreted by the parser according to the character set given by character_set_connection.

The following examples show that escape processsing occurs using character_set_connection even in the presence of an introducer. The examples use SET NAMES (which changes character_set_connection, as discussed in Section聽10.4, 鈥淐onnection Character Sets and Collations鈥�), and display the resulting strings using the HEX() function so that the exact string contents can be seen.

Example 1:

mysql> SET NAMES latin1;
Query OK, 0 rows affected (0.01 sec)

mysql> SELECT HEX('脿\n'), HEX(_sjis'脿\n');
+------------+-----------------+
| HEX('脿\n') | HEX(_sjis'脿\n') |
+------------+-----------------+
| E00A       | E00A            | 
+------------+-----------------+
1 row in set (0.00 sec)

Here, 鈥�脿鈥� (hex value E0) is followed by 鈥�\n鈥�, the escape sequence for newline. The escape sequence is interpreted using the character_set_connection value of latin1 to produce a literal newline (hex value 0A). This happens even for the second string. That is, the introducer of _sjis does not affect the parser's escape processing.

Example 2:

mysql> SET NAMES sjis;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT HEX('脿\n'), HEX(_latin1'脿\n');
+------------+-------------------+
| HEX('脿\n') | HEX(_latin1'脿\n') |
+------------+-------------------+
| E05C6E     | E05C6E            | 
+------------+-------------------+
1 row in set (0.04 sec)

Here, character_set_connection is sjis, a character set in which the sequence of 鈥�脿鈥� followed by 鈥�\鈥� (hex values 05 and 5C) is a valid multi-byte character. Hence, the first two bytes of the string are interpreted as a single sjis character, and the 鈥�\鈥� is not intrepreted as an escape character. The following 鈥�n鈥� (hex value 6E) is not interpreted as part of an escape sequence. This is true even for the second string; the introducer of _latin1 does not affect escape processing.

Before MySQL 4.1, NCHAR and CHAR were synonymous. Standard SQL defines NCHAR or NATIONAL CHAR as a way to indicate that a CHAR column should use some predefined character set. MySQL 4.1 and up uses utf8 as that predefined character set. For example, these data type declarations are equivalent:

CHAR(10) CHARACTER SET utf8
NATIONAL CHARACTER(10)
NCHAR(10)

As are these:

VARCHAR(10) CHARACTER SET utf8
NATIONAL VARCHAR(10)
NCHAR VARCHAR(10)
NATIONAL CHARACTER VARYING(10)
NATIONAL CHAR VARYING(10)

You can use N'literal' to create a string in the national character set. These two statements are equivalent:

SELECT N'some text';
SELECT _utf8'some text';

The following examples show how MySQL determines default character set and collation values.

Example 1: Table and Column Definition

CREATE TABLE t1
(
    c1 CHAR(10) CHARACTER SET latin1 COLLATE latin1_german1_ci
) DEFAULT CHARACTER SET latin2 COLLATE latin2_bin;

Here we have a column with a latin1 character set and a latin1_german1_ci collation. The definition is explicit, so that's straightforward. Notice that there is no problem with storing a latin1 column in a latin2 table.

Example 2: Table and Column Definition

CREATE TABLE t1
(
    c1 CHAR(10) CHARACTER SET latin1
) DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

This time we have a column with a latin1 character set and a default collation. Although it might seem natural, the default collation is not taken from the table level. Instead, because the default collation for latin1 is always latin1_swedish_ci, column c1 has a collation of latin1_swedish_ci (not latin1_danish_ci).

Example 3: Table and Column Definition

CREATE TABLE t1
(
    c1 CHAR(10)
) DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

We have a column with a default character set and a default collation. In this circumstance, MySQL checks the table level to determine the column character set and collation. Consequently, the character set for column c1 is latin1 and its collation is latin1_danish_ci.

Example 4: Database, Table, and Column Definition

CREATE DATABASE d1
    DEFAULT CHARACTER SET latin2 COLLATE latin2_czech_ci;
USE d1;
CREATE TABLE t1
(
    c1 CHAR(10)
);

We create a column without specifying its character set and collation. We're also not specifying a character set and a collation at the table level. In this circumstance, MySQL checks the database level to determine the table settings, which thereafter become the column settings.) Consequently, the character set for column c1 is latin2 and its collation is latin2_czech_ci.

For MaxDB compatibility these two statements are the same:

CREATE TABLE t1 (f1 CHAR(N) UNICODE);
CREATE TABLE t1 (f1 CHAR(N) CHARACTER SET ucs2);

With the COLLATE clause, you can override whatever the default collation is for a comparison. COLLATE may be used in various parts of SQL statements. Here are some examples:

The COLLATE clause has high precedence (higher than ||), so the following two expressions are equivalent:

x || y COLLATE z
x || (y COLLATE z)

The BINARY operator casts the string following it to a binary string. This is an easy way to force a comparison to be done byte by byte rather than character by character. BINARY also causes trailing spaces to be significant.

mysql> SELECT 'a' = 'A';
        -> 1
mysql> SELECT BINARY 'a' = 'A';
        -> 0
mysql> SELECT 'a' = 'a ';
        -> 1
mysql> SELECT BINARY 'a' = 'a ';
        -> 0

BINARY str is shorthand for CAST(str AS BINARY).

The BINARY attribute in character column definitions has a different effect. A character column defined with the BINARY attribute is assigned the binary collation of the column's character set. Every character set has a binary collation. For example, the binary collation for the latin1 character set is latin1_bin, so if the table default character set is latin1, these two column definitions are equivalent:

CHAR(10) BINARY
CHAR(10) CHARACTER SET latin1 COLLATE latin1_bin

The effect of BINARY as a column attribute differs from its effect prior to MySQL 4.1. Formerly, BINARY resulted in a column that was treated as a binary string. A binary string is a string of bytes that has no character set or collation, which differs from a non-binary character string that has a binary collation. For both types of strings, comparisons are based on the numeric values of the string unit, but for non-binary strings the unit is the character and some character sets allow multi-byte characters. Section聽11.4.2, 鈥淭he BINARY and VARBINARY Types鈥�.

The use of CHARACTER SET binary in the definition of a CHAR, VARCHAR, or TEXT column causes the column to be treated as a binary data type. For example, the following pairs of definitions are equivalent:

CHAR(10) CHARACTER SET binary
BINARY(10)

VARCHAR(10) CHARACTER SET binary
VARBINARY(10)

TEXT CHARACTER SET binary
BLOB

In the great majority of statements, it is obvious what collation MySQL uses to resolve a comparison operation. For example, in the following cases, it should be clear that the collation is the collation of column x:

SELECT x FROM T ORDER BY x;
SELECT x FROM T WHERE x = x;
SELECT DISTINCT x FROM T;

However, when multiple operands are involved, there can be ambiguity. For example:

SELECT x FROM T WHERE x = 'Y';

Should this query use the collation of the column x, or of the string literal 'Y'?

Standard SQL resolves such questions using what used to be called 鈥�coercibility鈥� rules. Basically, this means: Both x and 'Y' have collations, so which collation takes precedence? This can be difficult to resolve, but the following rules cover most situations:

The preceding coercibility values are current as of MySQL 4.1.11. See the note later in this section for additional version-related information.

Those rules resolve ambiguities in the following manner:

Examples:

The COERCIBILITY() function can be used to determine the coercibility of a string expression:

mysql> SELECT COERCIBILITY('A' COLLATE latin1_swedish_ci);
        -> 0
mysql> SELECT COERCIBILITY(VERSION());
        -> 3
mysql> SELECT COERCIBILITY('A');
        -> 4

See Section聽12.10.3, 鈥淚nformation Functions鈥�.

Before MySQL 4.1.11, there is no system constant or ignorable coercibility. Functions such as USER() have a coercibility of 2 rather than 3, and literals have a coercibility of 3 rather than 4.

Each character set has one or more collations, but each collation is associated with one and only one character set. Therefore, the following statement causes an error message because the latin2_bin collation is not legal with the latin1 character set:

mysql> SELECT _latin1 'x' COLLATE latin2_bin;
ERROR 1253 (42000): COLLATION 'latin2_bin' is not valid
for CHARACTER SET 'latin1'

In some cases, expressions that worked before MySQL 4.1 fail in early versions of MySQL 4.1 if you do not take character set and collation into account. For example, before 4.1, this statement works as is:

mysql> SELECT SUBSTRING_INDEX(USER(),'@',1);
+-------------------------------+
| SUBSTRING_INDEX(USER(),'@',1) |
+-------------------------------+
| root                          |
+-------------------------------+

The statement also works as is in MySQL 4.1 as of 4.1.8: In MySQL 4.1, usernames are stored using the utf8 character set (see Section聽10.8, 鈥淯TF-8 for Metadata鈥�). The literal string '@' has the server character set (latin1 by default). Although the character sets are different, MySQL can coerce the latin1 string to the character set (and collation) of USER() without data loss. It does so, performs the substring operation, and returns a result that has a character set of utf8.

However, in versions of MySQL 4.1 before 4.1.8, the statement fails:

mysql> SELECT SUBSTRING_INDEX(USER(),'@',1);
ERROR 1267 (HY000): Illegal mix of collations
(utf8_general_ci,IMPLICIT) and (latin1_swedish_ci,COERCIBLE)
for operation 'substr_index'

This happens because the automatic character set conversion of '@' does not occur and the string operands have different character sets (and thus different collations):

mysql> SELECT COLLATION(USER()), COLLATION('@');
+-------------------+-------------------+
| COLLATION(USER()) | COLLATION('@')    |
+-------------------+-------------------+
| utf8_general_ci   | latin1_swedish_ci |
+-------------------+-------------------+

One way to deal with this is to upgrade to MySQL 4.1.8 or later. If that is not possible, you can tell MySQL to interpret the literal string as utf8:

mysql> SELECT SUBSTRING_INDEX(USER(),_utf8'@',1);
+------------------------------------+
| SUBSTRING_INDEX(USER(),_utf8'@',1) |
+------------------------------------+
| root                               |
+------------------------------------+

Another way is to change the connection character set and collation to utf8. You can do that with SET NAMES 'utf8' or by setting the character_set_connection and collation_connection system variables directly.

Suppose that column X in table T has these latin1 column values:

Muffler
M眉ller
MX Systems
MySQL

Suppose also that the column values are retrieved using the following statement:

SELECT X FROM T ORDER BY X COLLATE collation_name;

The following table shows the resulting order of the values if we use ORDER BY with different collations:

The character that causes the different sort orders in this example is the U with two dots over it (眉), which the Germans call 鈥�U-umlaut.鈥�

MySQL has many operators and functions that return a string. This section answers the question: What is the character set and collation of such a string?

For simple functions that take string input and return a string result as output, the output's character set and collation are the same as those of the principal input value. For example, UPPER(X) returns a string whose character string and collation are the same as that of X. The same applies for INSTR(), LCASE(), LOWER(), LTRIM(), MID(), REPEAT(), REPLACE(), REVERSE(), RIGHT(), RPAD(), RTRIM(), SOUNDEX(), SUBSTRING(), TRIM(), UCASE(), and UPPER().

Note: The REPLACE() function, unlike all other functions, always ignores the collation of the string input and performs a case-sensitive comparison.

If a string input or function result is a binary string, the string has no character set or collation. This can be check by using the CHARSET() and COLLATION() functions, both of which return binary to indicate that their argument is a binary string:

mysql> SELECT CHARSET(BINARY 'a'), COLLATION(BINARY 'a');
+---------------------+-----------------------+
| CHARSET(BINARY 'a') | COLLATION(BINARY 'a') |
+---------------------+-----------------------+
| binary              | binary                |
+---------------------+-----------------------+

For operations that combine multiple string inputs and return a single string output, the 鈥�aggregation rules鈥� of standard SQL apply for determining the collation of the result:

For example, with CASE ... WHEN a THEN b WHEN b THEN c COLLATE X END, the resulting collation is X. The same applies for UNION, ||, CONCAT(), ELT(), GREATEST(), IF(), and LEAST().

For operations that convert to character data, the character set and collation of the strings that result from the operations are defined by the character_set_connection and collation_connection system variables. This applies only to CAST(), CHAR(), CONV(), FORMAT(), HEX(), and SPACE().

If you are uncertain about the character set or collation of the result returned by a string function, you can use the CHARSET() or COLLATE() function to find out:

mysql> SELECT USER(), CHARSET(USER()), COLLATION(USER());
+----------------+-----------------+-------------------+
| USER()         | CHARSET(USER()) | COLLATION(USER()) |
+----------------+-----------------+-------------------+
| test@localhost | utf8            | utf8_general_ci   | 
+----------------+-----------------+-------------------+

CONVERT() provides a way to convert data between different character sets. The syntax is:

CONVERT(expr USING transcoding_name)

In MySQL, transcoding names are the same as the corresponding character set names.

Examples:

SELECT CONVERT(_latin1'M眉ller' USING utf8);
INSERT INTO utf8table (utf8column)
    SELECT CONVERT(latin1field USING utf8) FROM latin1table;

CONVERT(... USING ...) is implemented according to the standard SQL specification.

You may also use CAST() to convert a string to a different character set. The syntax is:

CAST(character_string AS character_data_type CHARACTER SET charset_name)

Example:

SELECT CAST(_latin1'test' AS CHAR CHARACTER SET utf8);

If you use CAST() without specifying CHARACTER SET, the resulting character set and collation are defined by the character_set_connection and collation_connection system variables. If you use CAST() with CHARACTER SET X, the resulting character set and collation are X and the default collation of X.

You may not use a COLLATE clause inside a CAST(), but you may use it outside. That is, CAST(... COLLATE ...) is illegal, but CAST(...) COLLATE ... is legal.

Example:

SELECT CAST(_latin1'test' AS CHAR CHARACTER SET utf8) COLLATE utf8_bin;

Several SHOW statements are added or modified in MySQL 4.1 to provide additional character set information. SHOW CHARACTER SET, SHOW COLLATION, and SHOW CREATE DATABASE are new. SHOW CREATE TABLE and SHOW COLUMNS are modified. These statements are described here briefly. For more information, see Section聽13.5.4, 鈥�SHOW Syntax鈥�.

The SHOW CHARACTER SET command shows all available character sets. It takes an optional LIKE clause that indicates which character set names to match. For example:

mysql> SHOW CHARACTER SET LIKE 'latin%';
+---------+-----------------------------+-------------------+--------+
| Charset | Description                 | Default collation | Maxlen |
+---------+-----------------------------+-------------------+--------+
| latin1  | cp1252 West European        | latin1_swedish_ci |      1 |
| latin2  | ISO 8859-2 Central European | latin2_general_ci |      1 |
| latin5  | ISO 8859-9 Turkish          | latin5_turkish_ci |      1 |
| latin7  | ISO 8859-13 Baltic          | latin7_general_ci |      1 |
+---------+-----------------------------+-------------------+--------+

The output from SHOW COLLATION includes all available character sets. It takes an optional LIKE clause that indicates which collation names to match. For example:

mysql> SHOW COLLATION LIKE 'latin1%';
+-------------------+---------+----+---------+----------+---------+
| Collation         | Charset | Id | Default | Compiled | Sortlen |
+-------------------+---------+----+---------+----------+---------+
| latin1_german1_ci | latin1  |  5 |         |          |       0 |
| latin1_swedish_ci | latin1  |  8 | Yes     | Yes      |       0 |
| latin1_danish_ci  | latin1  | 15 |         |          |       0 |
| latin1_german2_ci | latin1  | 31 |         | Yes      |       2 |
| latin1_bin        | latin1  | 47 |         | Yes      |       0 |
| latin1_general_ci | latin1  | 48 |         |          |       0 |
| latin1_general_cs | latin1  | 49 |         |          |       0 |
| latin1_spanish_ci | latin1  | 94 |         |          |       0 |
+-------------------+---------+----+---------+----------+---------+

SHOW CREATE DATABASE displays the CREATE DATABASE statement that creates a given database:

mysql> SHOW CREATE DATABASE test;
+----------+-----------------------------------------------------------------+
| Database | Create Database                                                 |
+----------+-----------------------------------------------------------------+
| test     | CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET latin1 */ |
+----------+-----------------------------------------------------------------+

If no COLLATE clause is shown, the default collation for the character set applies.

SHOW CREATE TABLE is similar, but displays the CREATE TABLE statement to create a given table. The column definitions indicate any character set specifications, and the table options include character set information.

The SHOW COLUMNS statement displays the collations of a table's columns when invoked as SHOW FULL COLUMNS. Columns with CHAR, VARCHAR, or TEXT data types have collations. Numeric and other non-character types have no collation (indicated by NULL as the Collation value). For example:

mysql> SHOW FULL COLUMNS FROM person\G
*************************** 1. row ***************************
     Field: id
      Type: smallint(5) unsigned
 Collation: NULL
      Null: NO
       Key: PRI
   Default: NULL
     Extra: auto_increment
Privileges: select,insert,update,references
   Comment:
*************************** 2. row ***************************
     Field: name
      Type: char(60)
 Collation: latin1_swedish_ci
      Null: NO
       Key:
   Default:
     Extra:
Privileges: select,insert,update,references
   Comment:

The character set is not part of the display but is implied by the collation name.

Normally, the server runs using the latin1 character set by default. If you have been storing column data that actually is in some other character set that the 4.1 server supports directly, you can convert the column. However, you should avoid trying to convert directly from latin1 to the "real" character set. This may result in data loss. Instead, convert the column to a binary data type, and then from the binary type to a non-binary type with the desired character set. Conversion to and from binary involves no attempt at character value conversion and preserves your data intact. For example, suppose that you have a 4.0 table with three columns that are used to store values represented in latin1, latin2, and utf8:

CREATE TABLE t
(
    latin1_col CHAR(50),
    latin2_col CHAR(100),
    utf8_col CHAR(150)
);

For MySQL 4.1, you want to convert this table to leave latin1_col alone but change the latin2_col and utf8_col columns to have character sets of latin2 and utf8. Before upgrading to 4.1, back up your table, then convert the columns as follows:

ALTER TABLE t MODIFY latin2_col BINARY(100);
ALTER TABLE t MODIFY utf8_col BINARY(150);

Then, after upgrading to 4.1, complete the conversion by issuing these statements:

ALTER TABLE t MODIFY latin2_col CHAR(100) CHARACTER SET latin2;
ALTER TABLE t MODIFY utf8_col CHAR(150) CHARACTER SET utf8;

The first two statements 鈥�remove鈥� the character set information from the latin2_col and utf8_col columns. The second two statements assign the proper character sets to the two columns.

If you like, you can combine the to-binary conversions and from-binary conversions into single statements. In MySQL 4.0, do this:

ALTER TABLE t
    MODIFY latin2_col BINARY(100),
    MODIFY utf8_col BINARY(150);

After upgrading to 4.1, do this:

ALTER TABLE t
    MODIFY latin2_col CHAR(100) CHARACTER SET latin2,
    MODIFY utf8_col CHAR(150) CHARACTER SET utf8;

If you can ensure that the tables will not otherwise be modified before you perform the character set conversion, you can issue all of the ALTER TABLE statements after upgrading to MySQL 4.1.

If you specified attributes when creating a column initially, you should also specify them when altering the table with ALTER TABLE. For example, if you specified NOT NULL and an explicit DEFAULT value, you should also provide them in the ALTER TABLE statement. Otherwise, the resulting column definition will not include those attributes.

MySQL has two Unicode character sets. You can store text in about 650 languages using these character sets.

The MySQL implementation of UCS-2 stores characters in big-endian byte order and does not use a byte order mark (BOM) at the beginning of UCS-2 values. Other database systems might use little-ending byte order or a BOM, in which case conversion of UCS-2 values will need to be performed when transferring data between those systems and MySQL.

Note that in the ucs2_roman_ci and utf8_roman_ci collations, I and J are given the same precedence, and U and V are given the same precedence.

There is a limitation in MySQL 4.1 that results in two characters not being correctly handled when a user tries to change their case using LOWER() or UPPER():

Here are two workarounds for MySQL 4.1:

MySQL implements the utf8_unicode_ci collation 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 following discussion uses utf8_unicode_ci, but it is also true for ucs2_unicode_ci.

Currently, the utf8_unicode_ci collation has 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.

The most significant feature in utf8_unicode_ci is that it supports expansions; that is, when one character compares as equal to combinations of other characters. For example, in German and some other languages 鈥�脽鈥� is equal to 鈥�ss鈥�.

utf8_general_ci is a legacy collation that does not support expansions. It can make only one-to-one comparisons between characters. This means that comparisons for the utf8_general_ci collation are faster, but slightly less correct, than comparisons for utf8_unicode_ci.

For example, the following equalities hold in both utf8_general_ci and utf8_unicode_ci:

脛 = A
脰 = O
脺 = U

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

脽 = s

Whereas this is true for utf8_unicode_ci:

脽 = 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 and French, so there is no need to create special utf8 collations for these two languages.

utf8_general_ci also is satisfactory for both German and French, except that 鈥�脽鈥� is equal to 鈥�s鈥�, and not to 鈥�ss鈥�. If this is acceptable for your application, then you should use utf8_general_ci because it is faster. Otherwise, use utf8_unicode_ci because it is more accurate.

utf8_swedish_ci, like other utf8 language-specific collations, is derived from utf8_unicode_ci with additional language rules. For example, in Swedish, the following relationship holds, which is not something expected by a German or French speaker:

脺 = Y < 脰

The utf8_spanish_ci and utf8_spanish2_ci collations correspond to modern Spanish and traditional Spanish, respectively. In both collations, 鈥�帽鈥� (n-tilde) is a separate letter between 鈥�n鈥� and 鈥�o鈥�. In addition, for traditional Spanish, 鈥�ch鈥� is a separate letter between 鈥�c鈥� and 鈥�d鈥�, and 鈥�ll鈥� is a separate letter between 鈥�l鈥� and 鈥�m鈥�

Western European character sets cover most West European languages, such as French, Spanish, Catalan, Basque, Portuguese, Italian, Albanian, Dutch, German, Danish, Swedish, Norwegian, Finnish, Faroese, Icelandic, Irish, Scottish, and English.

MySQL provides some support for character sets used in the Czech Republic, Slovakia, Hungary, Romania, Slovenia, Croatia, Poland, and Serbia (Latin).

South European and Middle Eastern character sets supported by MySQL include Armenian, Arabic, Georgian, Greek, Hebrew, and Turkish.

The Baltic character sets cover Estonian, Latvian, and Lithuanian languages.

The Cyrillic character sets and collations are for use with Belarusian, Bulgarian, Russian, Ukrainian, and Serbian (Cyrillic) languages.

The Asian character sets that we support include Chinese, Japanese, Korean, and Thai. These can be complicated. For example, the Chinese sets must allow for thousands of different characters. See Section聽10.10.7.1, 鈥淭he cp932 Character Set鈥�, for additional information about the cp932 and sjis character sets.