MySQL Character Set Support

This chapter is from the book

MySQL Language Reference

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This chapter is from the book

This chapter is from the book 

MySQL Language Reference

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3.3 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.

3.3.1 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:

• According to the option settings in effect when the server starts

• According to the values set at runtime

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 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 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 are available as the values of the character_set_server and collation_server system variables. These variables can be changed at runtime.

3.3.2 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:

• If both CHARACTER SET X and COLLATE Y were specified, then character set X and collation Y.

• If CHARACTER SET X was specified without COLLATE, then character set X and its default collation.

• Otherwise, the server character set and server collation.

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.

3.3.3 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:

• If both CHARACTER SET X and COLLATE Y were specified, then character set X and collation Y.

• If CHARACTER SET X was specified without COLLATE, then character set X and its default collation.

• Otherwise, the database character set and collation.

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.

3.3.4 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:

• If both CHARACTER SET X and COLLATE Y were specified, then character set X and collation Y.

• If CHARACTER SET X was specified without COLLATE, then character set X and its default collation.

• Otherwise, the table character set and collation.

The CHARACTER SET and COLLATE clauses are standard SQL.

3.3.5 Examples of Character Set and Collation Assignment

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

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.

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).

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.

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.

3.3.6 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:

• The server character set and collation are available as the values of the character_set_server and collation_server variables.

• The character set and collation of the default database are available as the values of the character_set_database and collation_database variables.

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:

• What character set is the query in when it leaves the client?

The server takes the character_set_client variable to be the character set in which queries are sent by the client.

• What character set should the server translate a query to after receiving it?

For this, character_set_connection and collation_connection are used by the server. It converts queries sent by the client from character_set_client to character_set_ connection (except for string literals that have an introducer such as _latin1 or _utf8). collation_connection is important for comparisons of literal strings. For comparisons of strings with column values, it does not matter because columns have a higher collation precedence.

• What character set should the server translate to before shipping result sets or error messages back to the client?

The character_set_results variable indicates the character set in which the server returns query results to the client. This includes result data such as column values, and result metadata such as column names.

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 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 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 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;

3.3.7 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:

• If both _X and COLLATE Y were specified, then character set X and collation Y

• If _X is specified but COLLATE is not specified, then character set X and its default collation

• Otherwise, the character set and collation given by the character_set_connection and collation_connection system variables

Examples:

• A string with latin1 character set and latin1_german1_ci collation:

SELECT _latin1'Müller' COLLATE latin1_german1_ci;

• A string with latin1 character set and its default collation (that is, latin1_swedish_ci):

SELECT _latin1'Müller';

• A string with the connection default character set and collation:

SELECT 'Müller';

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

3.3.8 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:

• With ORDER BY:

SELECT k
FROM t1
ORDER BY k COLLATE latin1_german2_ci;

• With AS:

SELECT k COLLATE latin1_german2_ci AS k1
FROM t1
ORDER BY k1;

• With GROUP BY:

SELECT k
FROM t1
GROUP BY k COLLATE latin1_german2_ci;

• With aggregate functions:

SELECT MAX(k COLLATE latin1_german2_ci)
FROM t1;

• With DISTINCT:

SELECT DISTINCT k COLLATE latin1_german2_ci
FROM t1;

• With WHERE:

SELECT *
FROM t1
WHERE _latin1 'Müller' COLLATE latin1_german2_ci = k;

• With HAVING:

SELECT k
FROM t1
GROUP BY k
HAVING k = _latin1 'Müller' COLLATE latin1_german2_ci;

3.3.9 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) 

3.3.10 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;

3.3.11 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:

• An explicit COLLATE clause has a coercibility of 0. (Not coercible at all.)

• A concatenation of two strings with different collations has a coercibility of 1.

• A column's collation has a coercibility of 2.

• A literal's collation has a coercibility of 3.

Those rules resolve ambiguities thus:

• Use the collation with the lowest coercibility value.

• If both sides have the same coercibility, then it is an error if the collations aren't the same.

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('A');
    -> 3

See Section 5.8.3, "Information Functions."

3.3.12 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 3.6, "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.

3.3.13 An Example of the Effect of Collation

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

Muffler
Müller
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:

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-dieresis.

• The first column shows the result of the SELECT using the Swedish/Finnish collating rule, which says that U-dieresis sorts with Y.

• The second column shows the result of the SELECT using the German DIN-1 rule, which says that U-dieresis sorts with U.

• The third column shows the result of the SELECT using the German DIN-2 rule, which says that U-dieresis sorts with UE.

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.