(mysql.info) charset-connection

 
 10.4 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 been mentioned
 in earlier sections:
 
    * The server character set and collation can be determined from the
      values of the `character_set_server' and `collation_server' system
      variables.
 
    * The character set and collation of the default database can be
      determined from the values of the `character_set_database' and
      `collation_database' system variables.
 
 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.
 
 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 statement in when it leaves the client?
 
      The server takes the `character_set_client' system 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 `character_set_connection' and
      `collation_connection' system variables. It converts statements
      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,
      `collation_connection' does 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?
 
      The `character_set_results' system 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 the rest of 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 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.)
 
 A `SET NAMES 'X'' statement is equivalent to these three statements:
 
      SET character_set_client = X;
      SET character_set_results = X;
      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 to `SET NAMES' 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:
 
      SET character_set_client = X;
      SET character_set_results = X;
      SET collation_connection = @@collation_database;
 
 Setting `collation_connection' also sets `character_set_connection' to
 the character set associated with the collation.
 
 When a client connects, it sends to the server the name of the
 character set that it wants to use. The server uses the name to set the
 `character_set_client', `character_set_results', and
 `character_set_connection' system variables. In effect, the server
 performs a `SET NAMES' operation using the character set name.
 
 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 invoke `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 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 if there are characters that are not in
 both character sets.
 
 If you do not want the server to perform any conversion of result sets,
 set `character_set_results' to `NULL':
 
      SET character_set_results = NULL;
 
 * Currently, UCS-2 cannot be used as a client character set,
 which means that `SET NAMES 'ucs2'' does not work.
 
 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%';
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