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10.4 Connection Character Sets and Collations

Inhoudsopgave

A “connection” is what a client program makes when it connects to the server, to begin a session within which it interacts with the server. The client sends SQL statements, such as queries, over the session connection. The server sends responses, such as result sets or error messages, over the connection back to the client.

Connection Character Set and Collation System Variables

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

Additional character set and collation system variables are involved in handling traffic for the connection between a client and the server. Every client has session-specific connection-related character set and collation system variables. These session system variable values are initialized at connect time, but can be changed within the session.

Several questions about character set and collation handling for client connections can be answered in terms of system variables:

To see the values of the character set and collation system variables that apply to the current session, use this statement:

  1. SELECT * FROM performance_schema.session_variables
  2. WHERE VARIABLE_NAME IN (
  3. 'character_set_client', 'character_set_connection',
  4. 'character_set_results', 'collation_connection'
  5. ) ORDER BY VARIABLE_NAME;

The following simpler statements also display the connection variables, but include other related variables as well. They can be useful to see all character set and collation system variables:

  1. SHOW SESSION VARIABLES LIKE 'character\_set\_%';
  2. SHOW SESSION VARIABLES LIKE 'collation\_%';

Clients can fine-tune the settings for these variables, or depend on the defaults (in which case, you can skip the rest of this section). If you do not use the defaults, you must change the character settings for each connection to the server.

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Impermissible Client Character Sets

The character_set_client system variable cannot be set to certain character sets: 

ucs2
utf16
utf16le
utf32

Attempting to use any of those character sets as the client character set produces an error:

  1. mysql> SET character_set_client = 'ucs2';
  2. ERROR 1231 (42000): Variable 'character_set_client'
  3. can't be set to the value of 'ucs2'

The same error occurs if any of those character sets are used in the following contexts, all of which result in an attempt to set character_set_client to the named character set:

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Client Program Connection Character Set Configuration

When a client connects to the server, it indicates which character set it wants to use for communication with the server. (Actually, the client indicates the default collation for that character set, from which the server can determine the character set.) The server uses this information to set the character_set_client, character_set_results, character_set_connection system variables to the character set, and collation_connection to the character set default collation. In effect, the server performs the equivalent of a SET NAMES operation.

If the server does not support the requested character set or collation, it falls back to using the server character set and collation to configure the connection. For additional detail about this fallback behavior, see Connection Character Set Error Handling.

The mysql, mysqladmin, mysqlcheck, mysqlimport, and mysqlshow client programs determine the default character set to use as follows:

  • In the absence of other information, each client uses the compiled-in default character set, usually utf8mb4.

  • Each client can autodetect which character set to use based on the operating system setting, such as the value of the LANG or LC_ALL locale environment variable on Unix systems or the code page setting on Windows systems. For systems on which the locale is available from the OS, the client uses it to set the default character set rather than using the compiled-in default. For example, setting LANG to ru_RU.KOI8-R causes the koi8r character set to be used. Thus, users can configure the locale in their environment for use by MySQL clients.

    The OS character set is mapped to the closest MySQL character set if there is no exact match. If the client does not support the matching character set, it uses the compiled-in default. For example, utf8 and utf-8 map to utf8mb4, and ucs2 is not supported as a connection character set, so it maps to the compiled-in default.

    C applications can use character set autodetection based on the OS setting by invoking mysql_options() as follows before connecting to the server: 

    mysql_options(mysql,
              MYSQL_SET_CHARSET_NAME,
              MYSQL_AUTODETECT_CHARSET_NAME);

  • Each client supports a --default-character-set option, which enables users to specify the character set explicitly to override whatever default the client otherwise determines.

    Note

    Some character sets cannot be used as the client character set. Attempting to use them with --default-character-set produces an error. See Impermissible Client Character Sets.

With the mysql client, to use a character set different from the default, you could explicitly execute a SET NAMES statement every time you connect to the server (see Client Program Connection Character Set Configuration). To accomplish the same result more easily, specify the character set in your option file. For example, the following option file setting changes the three connection-related character set system variables set to koi8r each time you invoke mysql: 

[mysql]
default-character-set=koi8r

If you are using the mysql client with auto-reconnect enabled (which is not recommended), it is preferable to use the charset command rather than SET NAMES. For example:

  1. mysql> charset koi8r
  2. Charset changed

The charset command issues a SET NAMES statement, and also changes the default character set that mysql uses when it reconnects after the connection has dropped.

When configuration client programs, you must also consider the environment within which they execute. See Section 10.5, “Configuring Application Character Set and Collation”.

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SQL Statements for Connection Character Set Configuration

After a connection has been established, clients can change the character set and collation system variables for the current session. These variables can be changed individually using SET statements, but two more convenient statements affect the connection-related character set sytem variables as a group:

  • SET NAMES 'charset_name' [COLLATE 'collation_name']

    SET NAMES indicates what character set the client will use to send SQL statements to the server. Thus, SET NAMES 'cp1251' tells the server, “future incoming messages from this client are in character set cp1251.” It also specifies the character set that the server should use for sending results back to the client. (For example, it indicates what character set to use for column values if you use a SELECT statement that produces a result set.)

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

    1. SET character_set_client = charset_name;
    2. SET character_set_results = charset_name;
    3. SET character_set_connection = charset_name;

    Setting character_set_connection to charset_name also implicitly sets collation_connection to the default collation for charset_name. It is unnecessary to set that collation explicitly. To specify a particular collation to use for collation_connection, add a COLLATE clause:

    1. SET NAMES 'charset_name' COLLATE 'collation_name'

  • SET CHARACTER SET 'charset_name'

    SET CHARACTER SET is similar to SET NAMES but sets character_set_connection and collation_connection to character_set_database and collation_database (which, as mentioned previously, indicate the character set and collation of the default database).

    A SET CHARACTER SET charset_name statement is equivalent to these three statements:

    1. SET character_set_client = charset_name;
    2. SET character_set_results = charset_name;
    3. SET collation_connection = @@collation_database;

    Setting collation_connection also implicitly sets character_set_connection to the character set associated with the collation (equivalent to executing SET character_set_connection = @@character_set_database). It is unnecessary to set character_set_connection explicitly.

Note

Some character sets cannot be used as the client character set. Attempting to use them with SET NAMES or SET CHARACTER SET produces an error. See Impermissible Client Character Sets.

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 sends 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' before issuing the SELECT statement, the server converts the latin2 values to latin1 just before sending results back. Conversion may be lossy for characters that are not in both character sets.

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Connection Character Set Error Handling

Attempts to use an inappropriate connection character set or collation can produce an error, or cause the server to fall back to its default character set and collation for a given connection. This section describes problems that can occur when configuring the connection character set. These problems can occur when establishing a connection or when changing the character set within an established connection.

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Connect-Time Error Handling

Some character sets cannot be used as the client character set; see Impermissible Client Character Sets. If you specify a character set that is valid but not permitted as a client character set, the server returns an error: 

shell> mysql --default-character-set=ucs2
ERROR 1231 (42000): Variable 'character_set_client' can't be set to
the value of 'ucs2'

If you specify a character set that the client does not recognize, it produces an error: 

shell> mysql --default-character-set=bogus
mysql: Character set 'bogus' is not a compiled character set and is
not specified in the '/usr/local/mysql/share/charsets/Index.xml' file
ERROR 2019 (HY000): Can't initialize character set bogus
(path: /usr/local/mysql/share/charsets/)

If you specify a character set that the client recognizes but the server does not, the server falls back to its default character set and collation. Suppose that the server is configured to use latin1 and latin1_swedish_ci as its defaults, and that it does not recognize gb18030 as a valid character set. A client that specifies --default-character-set=gb18030 is able to connect to the server, but the resulting character set is not what the client wants:

  1. mysql> SHOW SESSION VARIABLES LIKE 'character\_set\_%';
  2. +--------------------------+--------+
  3. | Variable_name            | Value  |
  4. +--------------------------+--------+
  5. | character_set_client     | latin1 |
  6. | character_set_connection | latin1 |
  7. ...
  8. | character_set_results    | latin1 |
  9. ...
  10. +--------------------------+--------+
  11. mysql> SHOW SESSION VARIABLES LIKE 'collation_connection';
  12. +----------------------+-------------------+
  13. | Variable_name        | Value             |
  14. +----------------------+-------------------+
  15. | collation_connection | latin1_swedish_ci |
  16. +----------------------+-------------------+

You can see that the connection system variables have been set to reflect a character set and collation of latin1 and latin1_swedish_ci. This occurs because the server cannot satisfy the client character set request and falls back to its defaults.

In this case, the client cannot use the character set that it wants because the server does not support it. The client must either be willing to use a different character set, or connect to a different server that supports the desired character set.

The same problem occurs in a more subtle context: When the client tells the server to use a character set that the server recognizes, but the default collation for that character set on the client side is not known on the server side. This occurs, for example, when a MySQL 8.0 client wants to connect to a MySQL 5.7 server using utf8mb4 as the client character set. A client that specifies --default-character-set=utf8mb4 is able to connect to the server. However, as in the previous example, the server falls back to its default character set and collation, not what the client requested:

  1. mysql> SHOW SESSION VARIABLES LIKE 'character\_set\_%';
  2. +--------------------------+--------+
  3. | Variable_name            | Value  |
  4. +--------------------------+--------+
  5. | character_set_client     | latin1 |
  6. | character_set_connection | latin1 |
  7. ...
  8. | character_set_results    | latin1 |
  9. ...
  10. +--------------------------+--------+
  11. mysql> SHOW SESSION VARIABLES LIKE 'collation_connection';
  12. +----------------------+-------------------+
  13. | Variable_name        | Value             |
  14. +----------------------+-------------------+
  15. | collation_connection | latin1_swedish_ci |
  16. +----------------------+-------------------+

Why does this occur? After all, utf8mb4 is known to the 8.0 client and the 5.7 server, so both of them recognize it. To understand this behavior, it is necessary to understand that when the client tells the server which character set it wants to use, it really tells the server the default collation for that character set. Therefore, the aforementioned behavior occurs due to a combination of factors:

  • The default collation for utf8mb4 differs between MySQL 5.7 and 8.0 (utf8mb4_general_ci for 5.7, utf8mb4_0900_ai_ci for 8.0).

  • When the 8.0 client requests a character set of utf8mb4, what it sends to the server is the default 8.0 utf8mb4 collation; that is, the utf8mb4_0900_ai_ci.

  • utf8mb4_0900_ai_ci is implemented only as of MySQL 8.0, so the 5.7 server does not recognize it.

  • Because the 5.7 server does not recognize utf8mb4_0900_ai_ci, it cannot satisfy the client character set request, and falls back to its default character set and collation (latin1 and latin1_swedish_ci).

In this case, the client can still use utf8mb4 by issuing a SET NAMES 'utf8mb4' statement after connecting. The resulting collation is the 5.7 default utf8mb4 collation; that is, utf8mb4_general_ci. If the client additionally wants a collation of utf8mb4_0900_ai_ci, it cannot achieve that because the server does not recognize that collation. The client must either be willing to use a different utf8mb4 collation, or connect to a server from MySQL 8.0 or higher.

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Runtime Error Handling

Within an established connection, the client can request a change of connection character set and collation with SET NAMES or SET CHARACTER SET.

Some character sets cannot be used as the client character set; see Impermissible Client Character Sets. If you specify a character set that is valid but not permitted as a client character set, the server returns an error:

  1. mysql> SET NAMES 'ucs2';
  2. ERROR 1231 (42000): Variable 'character_set_client' can't be set to
  3. the value of 'ucs2'

If the server does not recognize the character set (or the collation), it produces an error:

  1. mysql> SET NAMES 'bogus';
  2. ERROR 1115 (42000): Unknown character set: 'bogus'
  3.  
  4. mysql> SET NAMES 'utf8mb4' COLLATE 'bogus';
  5. ERROR 1273 (HY000): Unknown collation: 'bogus'
Tip

A client that wants to verify whether its requested character set was honored by the server can execute the following statement after connecting and checking that the result is the expected character set:

  1. SELECT @@character_set_client;

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