Pre-General Availability Draft: 2018-02-21
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 connection-related character set and collation system variables.
A “connection” is 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 or error messages, 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 statement in when it leaves the client?
The server takes the
character_set_clientsystem variable to be the character set in which statements are sent by the client.
What character set should the server translate a statement to after receiving it?
For this, the server uses the
collation_connectionsystem variables. It converts statements sent by the client from
character_set_connection, except for string literals that have an introducer (for example,
collation_connectionis important for comparisons of literal strings. For comparisons of strings with column values,
collation_connectiondoes not matter because columns have their own collation, which has a higher collation precedence.
What character set should the server translate to before shipping result sets or error messages back to the client?
character_set_resultssystem 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 and error messages.
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.
Two statements affect the connection-related character set variables as a group:
SET NAMES '
charset_name' [COLLATE '
SET NAMESindicates 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
SET NAMES 'statement is equivalent to these three statements:
SET character_set_client = charset_name; SET character_set_results = charset_name; SET character_set_connection = charset_name;
charset_namealso implicitly sets
collation_connectionto the default collation for
charset_name. It is unnecessary to set that collation explicitly. To specify a particular collation, use the optional
SET NAMES 'charset_name' COLLATE 'collation_name'
SET CHARACTER SET ''
SET CHARACTER SETis similar to
SET NAMESbut sets
SET CHARACTER SETstatement is equivalent to these three statements:
SET character_set_client = charset_name; SET character_set_results = charset_name; SET collation_connection = @@collation_database;
collation_connectionalso implicitly sets
character_set_connectionto the character set associated with the collation (equivalent to executing
SET character_set_connection = @@character_set_database). It is unnecessary to set
In the absence of other information, the programs use the compiled-in default character set, usually
The programs can autodetect which character set to use based on the operating system setting, such as the value of the
LC_ALLlocale 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
koi8rcharacter 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,
ucs2is not supported as a connection character set.
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);
The programs support a
--default-character-setoption, which enables users to specify the character set explicitly to override whatever default the client otherwise determines.
When a client connects to the server, it sends the name of the
character set to be used for communication with the server. The
server uses the name to set the
variables. In effect, the server performs a
SET NAMES operation using the
character set name.
With the mysql client, to use a character set
different from the default, you could explicitly execute
SET NAMES every time you start up.
To accomplish the same result more easily, add the
setting to your mysql command line or in your
option file. For example, the following option file setting
changes the three connection-related character set variables set
koi8r each time you invoke
mysql> charset utf8 Charset changed
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
latin2 values to
latin1 just before sending results back.
Conversion may be lossy if there are characters that are not in
both character sets.
If you want the server to perform no conversion of result sets or
error messages, set
SET character_set_results = NULL; SET character_set_results = binary;
To see the values of the character set and collation system variables that apply to your connection, use these statements:
SHOW VARIABLES LIKE 'character_set%'; SHOW VARIABLES LIKE 'collation%';
You must also consider the environment within which your MySQL applications execute. See Section 10.5, “Configuring Application Character Set and Collation”.
For more information about character sets and error messages, see Section 10.6, “Error Message Character Set”.