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11 Character Set Support

Improved support for character set handling was added to MySQL in Version 4.1. The features described here are as implemented in MySQL 4.1.1. (MySQL 4.1.0 has some but not all of these features, and some of them are implemented differently.)

This chapter discusses the following topics:

Character set support currently is included in the MyISAM, MEMORY (HEAP), and (as of MySQL 4.1.2) InnoDB storage engines. The ISAM storage engine does not include character set support; there are no plans to change this, because ISAM is deprecated.

11.1 Character Sets and Collations in General

A character set is a set of symbols and encodings. A collation is a set of rules for comparing characters in a character set. Let's make the distinction clear with an example of an imaginary character set.

Suppose that we have an alphabet with four letters: `A', `B', `a', `b'. We give each letter a number: `A' = 0, `B' = 1, `a' = 2, `c' = 3. The letter `A' is a symbol, the number 0 is the encoding for `A', and the combination of all four letters and their encodings is a character set.

Now, suppose that we want to compare two string values, `A' and `B'. The simplest way to do this is to look at the encodings: 0 for `A' and 1 for `B'. Because 0 is less than 1, we say `A' is less than `B'. Now, what we've just done is apply a collation to our character set. The collation is a set of rules (only one rule in this case): ``compare the encodings.'' We call this simplest of all possible collations a binary collation.

But what if we want to say that the lowercase and uppercase letters are equivalent? Then we would have at least two rules: (1) treat the lowercase letters `a' and `b' as equivalent to `A' and `B'; (2) then compare the encodings. We call this a case-insensitive collation. It's a little more complex than a binary collation.

In real life, most character sets have many characters: not just `A' and `B' but whole alphabets, sometimes multiple alphabets or eastern writing systems with thousands of characters, along with many special symbols and punctuation marks. Also in real life, most collations have many rules: not just case insensitivity but also accent insensitivity (an ``accent'' is a mark attached to a character as in German `@"O') and multiple-character mappings (such as the rule that `@"O' = `OE' in one of the two German collations).

MySQL 4.1 can do these things for you:

In these respects, not only is MySQL 4.1 far more flexible than MySQL 4.0, it also is far ahead of other DBMSs. However, to use the new features effectively, you will need to learn what character sets and collations are available, how to change their defaults, and what the various string operators do with them.

11.2 Character Sets and Collations in MySQL

The MySQL server can support multiple character sets. To list the available character sets, use the SHOW CHARACTER SET statement: 

mysql> SHOW CHARACTER SET;
+----------+-----------------------------+---------------------+
| Charset  | Description                 | Default collation   |
+----------+-----------------------------+---------------------+
| big5     | Big5 Traditional Chinese    | big5_chinese_ci     |
| dec8     | DEC West European           | dec8_swedish_ci     |
| cp850    | DOS West European           | cp850_general_ci    |
| hp8      | HP West European            | hp8_english_ci      |
| koi8r    | KOI8-R Relcom Russian       | koi8r_general_ci    |
| latin1   | ISO 8859-1 West European    | latin1_swedish_ci   |
| latin2   | ISO 8859-2 Central European | latin2_general_ci   |
...

The output actually includes another column that is not shown so that the example fits better on the page.

Any given character set always has at least one collation. It may have several collations.

To list the collations for a character set, use the SHOW COLLATION statement. For example, to see the collations for the latin1 (``ISO-8859-1 West European'') character set, use this statement to find those collation names that begin with latin1: 

mysql> SHOW COLLATION LIKE 'latin1%';
+-------------------+---------+----+---------+----------+---------+
| Collation         | Charset | Id | Default | Compiled | Sortlen |
+-------------------+---------+----+---------+----------+---------+
| latin1_german1_ci | latin1  |  5 |         |          |       0 |
| latin1_swedish_ci | latin1  |  8 | Yes     | Yes      |       1 |
| latin1_danish_ci  | latin1  | 15 |         |          |       0 |
| latin1_german2_ci | latin1  | 31 |         | Yes      |       2 |
| latin1_bin        | latin1  | 47 |         | Yes      |       1 |
| latin1_general_ci | latin1  | 48 |         |          |       0 |
| latin1_general_cs | latin1  | 49 |         |          |       0 |
| latin1_spanish_ci | latin1  | 94 |         |          |       0 |
+-------------------+---------+----+---------+----------+---------+

The latin1 collations have the following meanings:

Collation Meaning
latin1_bin Binary according to latin1 encoding
latin1_danish_ci Danish/Norwegian
latin1_general_ci Multilingual
latin1_general_cs Multilingual, case sensitive
latin1_german1_ci German DIN-1
latin1_german2_ci German DIN-2
latin1_spanish_ci Modern Spanish
latin1_swedish_ci Swedish/Finnish

Collations have these general characteristics:

11.3 Determining the Default Character Set and Collation

There are default settings for character sets and collations at four levels: server, database, table, and connection. The following description may appear complex, but it has been found in practice that multiple-level defaulting leads to natural and obvious results.

11.3.1 Server Character Set and Collation

The MySQL Server has a server character set and a server collation, which may not be null.

MySQL determines the server character set and server collation thus:

At the server level, the decision is simple. The server character set and collation depend initially on the options that you use when you start @command{mysqld}. You can use --default-character-set for the character set, and along with it you can add --default-collation for the collation. If you don't specify a character set, that is the same as saying --default-character-set=latin1. If you specify only a character set (for example, latin1) but not a collation, that is the same as saying --default-charset=latin1 --default-collation=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 --default-character-set=latin1
shell> mysqld --default-character-set=latin1 \
           --default-collation=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 @command{configure}. For example: 

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

Or: 

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

Both @command{mysqld} and @command{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 are available as the values of the character_set_server and collation_server system variables. These variables can be changed at runtime.

11.3.2 Database Character Set and Collation

Every database has a database character set and a database collation, which may not be null. 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]

Example: 

CREATE DATABASE db_name
    DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_ci;

MySQL chooses the database character set and database collation thus:

MySQL's CREATE DATABASE ... DEFAULT CHARACTER SET ... syntax is analogous to the standard SQL CREATE SCHEMA ... CHARACTER SET ... syntax. Because of this, it is possible to create databases with different character sets and collations on the same MySQL server.

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 are available as 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 variables, character_set_server and collation_server.

11.3.3 Table Character Set and Collation

Every table has a table character set and a table collation, which may not be null. 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 ( ... )
    DEFAULT CHARACTER SET latin1 COLLATE latin1_danish_ci;

MySQL chooses the table character set and collation thus:

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.

11.3.4 Column Character Set and Collation

Every ``character'' column (that is, a column of type CHAR, VARCHAR, or TEXT) has a column character set and a column collation, which may not be null. 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 thus:

The CHARACTER SET and COLLATE clauses are standard SQL.

11.3.5 Examples of Character Set and Collation Assignment

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

11.3.5.1 Example 1: Table + 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's no problem storing a latin1 column in a latin2 table.

11.3.5.2 Example 2: Table + 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. Now, 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 will have a collation of latin1_swedish_ci (not latin1_danish_ci).

11.3.5.3 Example 3: Table + 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 looks up to the table level for inspiration in determining the column character set and collation. So, the character set for column c1 is latin1 and its collation is latin1_danish_ci.

11.3.5.4 Example 4: Database + Table + 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 looks up to the database level for inspiration. (The database's settings become the table's settings, and thereafter become the column's setting.) So, the character set for column c1 is latin2 and its collation is latin2_czech_ci.

11.3.6 Connection Character Sets and Collations

Several character set and collation system variables relate to a client's interaction with the server. Some of these have already been mentioned in earlier sections:

Additional character set and collation variables are involved in handling traffic for the connection between a client and the server. Every client has connection-related character set and collation variables.

Consider what a ``connection'' is: It's what you make when you connect to the server. The client sends SQL statements, such as queries, over the connection to the server. The server sends responses, such as result sets, over the connection back to the client. This leads to several questions about character set and collation handling for client connections, each of which can be answered in terms of system variables:

You can fine-tune the settings for these variables, or you can depend on the defaults (in which case, you can skip this section).

There are two statements that affect the connection character sets: 

SET NAMES 'charset_name'
SET CHARACTER SET charset_name

SET NAMES indicates what is in the SQL statements that the client sends. Thus, SET NAMES 'cp1251' tells the server ``future incoming messages from this client will be in character set cp1251.'' It also specifies the character set for results that the server sends back to the client. (For example, it indicates what character set column values will have if you use a SELECT statement.)

A SET NAMES 'x' statement is equivalent to these three statements: 

mysql> SET character_set_client = x;
mysql> SET character_set_results = x;
mysql> SET character_set_connection = x;

Setting character_set_connection to x also sets collation_connection to the default collation for x.

SET CHARACTER SET is similar but sets the connection character set and collation to be those of the default database. A SET CHARACTER SET x statement is equivalent to these three statements: 

mysql> SET character_set_client = x;
mysql> SET character_set_results = x;
mysql> SET collation_connection = @@collation_database;

When a client connects, it sends to the server the name of the character set that it wants to use. The server sets the character_set_client, character_set_results, and character_set_connection variables to that character set. (In effect, the server performs a SET NAMES operation using the character set.)

With the @command{mysql} client, it is not necessary to execute SET NAMES every time you start up if you want to use a character set different from the default. You can add the --default-character-set option setting to your @command{mysql} statement line, or in your option file. For example, the following option file setting changes the three character set variables set to koi8r each time you run @command{mysql}: 

[mysql]
default-character-set=koi8r

Example: Suppose that column1 is defined as CHAR(5) CHARACTER SET latin2. If you do not say SET NAMES or SET CHARACTER SET, then for SELECT column1 FROM t, the server will send back all the values for column1 using the character set that the client specified when it connected. On the other hand, if you say SET NAMES 'latin1' or SET CHARACTER SET latin1, then just before sending results back, the server will convert the latin2 values to latin1. Conversion may be lossy if there are characters that are not in both character sets.

If you do not want the server to perform any conversion, set character_set_results to NULL: 

mysql> SET character_set_results = NULL;

11.3.7 Character String Literal Character Set and Collation

Every character string literal has a character set and a collation, which may not be null.

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, ``the string that is about to follow is in character set X.'' 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), and before ? (parameter substitution when using prepared statements within a programming language interface).

Examples: 

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

MySQL determines a literal's character set and collation thus:

Examples:

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

11.3.8 Using COLLATE in SQL Statements

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:

11.3.9 COLLATE Clause Precedence

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

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

11.3.10 BINARY Operator

The BINARY operator is a shorthand for a COLLATE clause. BINARY 'x' is equivalent to 'x' COLLATE y, where y is the name of the binary collation for the character set of 'x'. Every character set has a binary collation. For example, the binary collation for the latin1 character set is latin1_bin, so if the column a is of character set latin1, the following two statements have the same effect: 

SELECT * FROM t1 ORDER BY BINARY a;
SELECT * FROM t1 ORDER BY a COLLATE latin1_bin;

11.3.11 Some Special Cases Where the Collation Determination Is Tricky

In the great majority of queries, 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 will be ``the column 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 ``coercibility'' rules. The essence is: Because x and 'Y' both have collations, whose collation takes precedence? It's complex, but the following rules take care of most situations:

Those rules resolve ambiguities thus:

Examples:

column1 = 'A' Use collation of column1
column1 = 'A' COLLATE x Use collation of 'A'
column1 COLLATE x = 'A' COLLATE y Error

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('A');
        -> 3

See section 13.8.3 Information Functions.

11.3.12 Collations Must Be for the Right Character Set

Recall that each character set has one or more collations, and 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 1251: COLLATION 'latin2_bin' is not valid
for CHARACTER SET 'latin1'

In some cases, expressions that worked before MySQL 4.1 fail as 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                          |
+-------------------------------+

After an upgrade to MySQL 4.1, 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'

The reason this occurs is that usernames are stored using UTF8 (see section 11.6 UTF8 for Metadata). As a result, the USER() function and the literal string '@' 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 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.

11.3.13 An Example of the Effect of Collation

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

Muffler
M@"uller
MX Systems
MySQL

And suppose that the column values are retrieved using the following statement: 

SELECT X FROM T ORDER BY X COLLATE collation_name;

The resulting order of the values for different collations is shown in this table:

latin1_swedish_ci latin1_german1_ci latin1_german2_ci
Muffler Muffler M@"uller
MX Systems M@"uller Muffler
M@"uller MX Systems MX Systems
MySQL MySQL MySQL

The table is an example that shows what the effect would be if we used different collations in an ORDER BY clause. The character that causes the different sort orders in this example is the U with two dots over it, which the Germans call U-umlaut, but we'll call it U-diaeresis.

Three different collations, three different results. That's what MySQL is here to handle. By using the appropriate collation, you can choose the sort order you want.

11.4 Operations Affected by Character Set Support

This section describes operations that take character set information into account as of MySQL 4.1.

11.4.1 Result Strings

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(). (Also note: The REPLACE() function, unlike all other functions, ignores the collation of the string input and performs a case-insensitive comparison every time.)

For operations that combine multiple string inputs and return a single string output, the ``aggregation rules'' of standard SQL apply:

For example, with CASE ... WHEN a THEN b WHEN b THEN c COLLATE X END, the resultant collation is X. The same applies for CASE, 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 for CAST(), CHAR(), CONV(), FORMAT(), HEX(), and SPACE().

11.4.2 CONVERT()

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@"uller' USING utf8);
INSERT INTO utf8table (utf8column)
    SELECT CONVERT(latin1field USING utf8) FROM latin1table;

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

11.4.3 CAST()

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, then 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;

11.4.4 SHOW Statements

Several SHOW statements are new 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.

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  | ISO 8859-1 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 |
+---------+-----------------------------+-------------------+--------+

See section 14.5.3.2 SHOW CHARACTER SET Syntax.

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 |
+-------------------+---------+----+---------+----------+---------+

See section 14.5.3.3 SHOW COLLATION Syntax.

SHOW CREATE DATABASE displays the CREATE DATABASE statement that will create a given database. The result includes all database options. DEFAULT CHARACTER SET and COLLATE are supported. All database options are stored in a text file named `db.opt' that can be found in the database directory. 

mysql> SHOW CREATE DATABASE a\G
*************************** 1. row ***************************
       Database: a
Create Database: CREATE DATABASE `a`
                 /*!40100 DEFAULT CHARACTER SET macce */

See section 14.5.3.5 SHOW CREATE DATABASE Syntax.

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

See section 14.5.3.6 SHOW CREATE TABLE Syntax.

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 non-NULL collations. Numeric and other non-character types have NULL collations. For example: 

mysql> SHOW FULL COLUMNS FROM t;
+-------+---------+------------+------+-----+---------+-------+
| Field | Type    | Collation  | Null | Key | Default | Extra |
+-------+---------+------------+------+-----+---------+-------+
| a     | char(1) | latin1_bin | YES  |     | NULL    |       |
| b     | int(11) | NULL       | YES  |     | NULL    |       |
+-------+---------+------------+------+-----+---------+-------+

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

See section 14.5.3.4 SHOW COLUMNS Syntax.

11.5 Unicode Support

As of MySQL version 4.1, there are two new character sets for storing Unicode data:

In UCS-2 (binary Unicode representation), every character is represented by a two-byte Unicode code with the most significant byte first. For example: "LATIN CAPITAL LETTER A" has the code 0x0041 and it's stored as a two-byte sequence: 0x00 0x41. "CYRILLIC SMALL LETTER YERU" (Unicode 0x044B) is stored as a two-byte sequence: 0x04 0x4B. For Unicode characters and their codes, please refer to the Unicode Home Page.

A temporary restriction is that UCS-2 cannot yet be used as a client character set. That means that SET NAMES 'ucs2' will not work.

The UTF8 character set (transform Unicode representation) is an alternative way to store Unicode data. It is implemented according to RFC2279. The idea of the UTF8 character set is that various Unicode characters fit into byte sequences of different lengths:

Currently, MySQL UTF8 support does not include four-byte sequences.

Tip: To save space with UTF8, use VARCHAR instead of CHAR. Otherwise, MySQL has to reserve 30 bytes for a CHAR(10) CHARACTER SET utf8 column, because that's the maximum possible length.

11.6 UTF8 for Metadata

The metadata is the data about the data. Anything that describes the database, as opposed to being the contents of the database, is metadata. Thus column names, database names, usernames, version names, and most of the string results from SHOW are metadata.

Representation of metadata must satisfy these requirements:

In order to satisfy both requirements, MySQL stores metadata in a Unicode character set, namely UTF8. This will not cause any disruption if you never use accented characters. But if you do, you should be aware that metadata is in UTF8.

This means that the USER(), CURRENT_USER(), and VERSION() functions will have the UTF8 character set by default. So will any synonyms, such the SESSION_USER() and SYSTEM_USER() synonyms for USER().

The server sets the character_set_system system variable to the name of the metadata character set: 

mysql> SHOW VARIABLES LIKE 'character_set_system';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| character_set_system | utf8  |
+----------------------+-------+

Storage of metadata using Unicode does not mean that the headers of columns and the results of DESCRIBE functions will be in the character_set_system character set by default. When you say SELECT column1 FROM t, the name column1 itself will be returned from the server to the client in the character set as determined by the SET NAMES statement. More specifically, the character set used is determined by the value of the character_set_results system variable. If this variable is set to NULL, no conversion is performed and the server returns metadata using its original character set (the set indicated by character_set_system).

If you want the server to pass metadata results back in a non-UTF8 character set, then use SET NAMES to force the server to perform character set conversion (see section 11.3.6 Connection Character Sets and Collations), or else set the client to do the conversion. It is always more efficient to set the client to do the conversion, but this option will not be available for many clients until late in the MySQL 4.x product cycle.

If you are just using, for example, the USER() function for comparison or assignment within a single statement, don't worry. MySQL will do some automatic conversion for you. 

SELECT * FROM Table1 WHERE USER() = latin1_column;

This will work because the contents of latin1_column are automatically converted to UTF8 before the comparison. 

INSERT INTO Table1 (latin1_column) SELECT USER();

This will work because the contents of USER() are automatically converted to latin1 before the assignment. Automatic conversion is not fully implemented yet, but should work correctly in a later version.

Although automatic conversion is not in the SQL standard, the SQL standard document does say that every character set is (in terms of supported characters) a ``subset'' of Unicode. Since it is a well-known principle that ``what applies to a superset can apply to a subset,'' we believe that a collation for Unicode can apply for comparisons with non-Unicode strings.

11.7 Compatibility with Other DBMSs

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);

11.8 New Character Set Configuration File Format

In MySQL 4.1, character set configuration is stored in XML files, one file per character set. In previous versions, this information was stored in `.conf' files.

11.9 National Character Set

Before MySQL 4.1, NCHAR and CHAR were synonymous. ANSI 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 column 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';

11.10 Upgrading Character Sets from MySQL 4.0

Now, what about upgrading from older versions of MySQL? MySQL 4.1 is almost upward compatible with MySQL 4.0 and earlier for the simple reason that almost all the features are new, so there's nothing in earlier versions to conflict with. However, there are some differences and a few things to be aware of.

Most important: The ``MySQL 4.0 character set'' has the properties of both ``MySQL 4.1 character sets'' and ``MySQL 4.1 collations.'' You will have to unlearn this. Henceforth, we will not bundle character set/collation properties in the same conglomerate object.

There is a special treatment of national character sets in MySQL 4.1. NCHAR is not the same as CHAR, and N'...' literals are not the same as '...' literals.

Finally, there is a different file format for storing information about character sets and collations. Make sure that you have reinstalled the `/share/mysql/charsets/' directory containing the new configuration files.

If you want to start @command{mysqld} from a 4.1.x distribution with data created by MySQL 4.0, you should start the server with the same character set and collation. In this case, you won't need to reindex your data.

There are two ways to do so: 

shell> ./configure --with-charset=... --with-collation=...
shell> ./mysqld --default-character-set=... --default-collation=...

If you used @command{mysqld} with, for example, the MySQL 4.0 danish character set, you should now use the latin1 character set and the latin1_danish_ci collation: 

shell> ./configure --with-charset=latin1 \
           --with-collation=latin1_danish_ci
shell> ./mysqld --default-character-set=latin1 \
           --default-collation=latin1_danish_ci

Use the table shown in section 11.10.1 4.0 Character Sets and Corresponding 4.1 Character Set/Collation Pairs to find old 4.0 character set names and their 4.1 character set/collation pair equivalents.

If you have non-latin1 data stored in a 4.0 latin1 table and want to convert the table column definitions to reflect the actual character set of the data, use the instructions in section 11.10.2 Converting 4.0 Character Columns to 4.1 Format.

11.10.1 4.0 Character Sets and Corresponding 4.1 Character Set/Collation Pairs

ID 4.0 Character Set 4.1 Character Set 4.1 Collation
1 big5 big5 big5_chinese_ci
2 czech latin2 latin2_czech_ci
3 dec8 dec8 dec8_swedish_ci
4 dos cp850 cp850_general_ci
5 german1 latin1 latin1_german1_ci
6 hp8 hp8 hp8_english_ci
7 koi8_ru koi8r koi8r_general_ci
8 latin1 latin1 latin1_swedish_ci
9 latin2 latin2 latin2_general_ci
10 swe7 swe7 swe7_swedish_ci
11 usa7 ascii ascii_general_ci
12 ujis ujis ujis_japanese_ci
13 sjis sjis sjis_japanese_ci
14 cp1251 cp1251 cp1251_bulgarian_ci
15 danish latin1 latin1_danish_ci
16 hebrew hebrew hebrew_general_ci
17 win1251 (removed) (removed)
18 tis620 tis620 tis620_thai_ci
19 euc_kr euckr euckr_korean_ci
20 estonia latin7 latin7_estonian_ci
21 hungarian latin2 latin2_hungarian_ci
22 koi8_ukr koi8u koi8u_ukrainian_ci
23 win1251ukr cp1251 cp1251_ukrainian_ci
24 gb2312 gb2312 gb2312_chinese_ci
25 greek greek greek_general_ci
26 win1250 cp1250 cp1250_general_ci
27 croat latin2 latin2_croatian_ci
28 gbk gbk gbk_chinese_ci
29 cp1257 cp1257 cp1257_lithuanian_ci
30 latin5 latin5 latin5_turkish_ci
31 latin1_de latin1 latin1_german2_ci

11.10.2 Converting 4.0 Character Columns to 4.1 Format

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 now 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 column 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)
);

After upgrading to 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. First, 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);
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 ``remove'' 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: 

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

11.11 Character Sets and Collations That MySQL Supports

Here is an annotated list of character sets and collations that MySQL supports. Because options and installation settings differ, some sites might not have all items listed, and some sites might have items not listed.

MySQL supports 70+ collations for 30+ character sets. The character sets and their default collations are displayed by the SHOW CHARACTER SET STATEMENT. (The output actually includes another column that is not shown so that the example fits better on the page.) 

mysql> SHOW CHARACTER SET;
+----------+-----------------------------+---------------------+
| Charset  | Description                 | Default collation   |
+----------+-----------------------------+---------------------+
| big5     | Big5 Traditional Chinese    | big5_chinese_ci     |
| dec8     | DEC West European           | dec8_swedish_ci     |
| cp850    | DOS West European           | cp850_general_ci    |
| hp8      | HP West European            | hp8_english_ci      |
| koi8r    | KOI8-R Relcom Russian       | koi8r_general_ci    |
| latin1   | ISO 8859-1 West European    | latin1_swedish_ci   |
| latin2   | ISO 8859-2 Central European | latin2_general_ci   |
| swe7     | 7bit Swedish                | swe7_swedish_ci     |
| ascii    | US ASCII                    | ascii_general_ci    |
| ujis     | EUC-JP Japanese             | ujis_japanese_ci    |
| sjis     | Shift-JIS Japanese          | sjis_japanese_ci    |
| cp1251   | Windows Cyrillic            | cp1251_bulgarian_ci |
| hebrew   | ISO 8859-8 Hebrew           | hebrew_general_ci   |
| tis620   | TIS620 Thai                 | tis620_thai_ci      |
| euckr    | EUC-KR Korean               | euckr_korean_ci     |
| koi8u    | KOI8-U Ukrainian            | koi8u_general_ci    |
| gb2312   | GB2312 Simplified Chinese   | gb2312_chinese_ci   |
| greek    | ISO 8859-7 Greek            | greek_general_ci    |
| cp1250   | Windows Central European    | cp1250_general_ci   |
| gbk      | GBK Simplified Chinese      | gbk_chinese_ci      |
| latin5   | ISO 8859-9 Turkish          | latin5_turkish_ci   |
| armscii8 | ARMSCII-8 Armenian          | armscii8_general_ci |
| utf8     | UTF-8 Unicode               | utf8_general_ci     |
| ucs2     | UCS-2 Unicode               | ucs2_general_ci     |
| cp866    | DOS Russian                 | cp866_general_ci    |
| keybcs2  | DOS Kamenicky Czech-Slovak  | keybcs2_general_ci  |
| macce    | Mac Central European        | macce_general_ci    |
| macroman | Mac West European           | macroman_general_ci |
| cp852    | DOS Central European        | cp852_general_ci    |
| latin7   | ISO 8859-13 Baltic          | latin7_general_ci   |
| cp1256   | Windows Arabic              | cp1256_general_ci   |
| cp1257   | Windows Baltic              | cp1257_general_ci   |
| binary   | Binary pseudo charset       | binary              |
| geostd8  | GEOSTD8 Georgian            | geostd8_general_ci  |
+----------+-----------------------------+---------------------+

11.11.1 Unicode Character Sets

MySQL has two Unicode character sets. You can store texts in about 650 languages using these character sets. We have added several collations for these two new sets, with more to come.

11.11.2 West European Character Sets

West 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.

11.11.3 Central European Character Sets

We have some support for character sets used in the Czech Republic, Slovakia, Hungary, Romania, Slovenia, Croatia, and Poland.

11.11.4 South European and Middle East Character Sets

11.11.5 Baltic Character Sets

The Baltic character sets cover Estonian, Latvian, and Lithuanian languages. There are two Baltic character sets currently supported:

11.11.6 Cyrillic Character Sets

Here are the Cyrillic character sets and collations for use with Belarusian, Bulgarian, Russian, and Ukrainian languages.

11.11.7 Asian Character Sets

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.

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