(mysql.info) charset-metadata

 
 10.8 UTF-8 for Metadata
 =======================
 
 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. This is also
 true of the contents of tables in `INFORMATION_SCHEMA', because those
 tables by definition contain information about database objects.
 
 Representation of metadata must satisfy these requirements:
 
    * All metadata must be in the same character set. Otherwise, neither
      the `SHOW' commands nor `SELECT' statements for tables in
      `INFORMATION_SCHEMA' would work properly because different rows in
      the same column of the results of these operations would be in
      different character sets.
 
    * Metadata must include all characters in all languages.  Otherwise,
      users would not be able to name columns and tables using their own
      languages.
 
 To satisfy both requirements, MySQL stores metadata in a Unicode
 character set, namely UTF-8. This does not cause any disruption if you
 never use accented or non-Latin characters. But if you do, you should
 be aware that metadata is in UTF-8.
 
 The metadata requirements mean that the return values of the `USER()',
 `CURRENT_USER()', `DATABASE()', and `VERSION()' functions have the
 UTF-8 character set by default, as do synonyms such as `SESSION_USER()'
 and `SYSTEM_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 server
 returns headers of columns and the results of `DESCRIBE' functions in
 the `character_set_system' character set by default.  When you use
 `SELECT column1 FROM t', the name `column1' itself is returned from the
 server to the client in the character set determined by the value of the
 `character_set_results' system variable, which has a default value of
 `latin1'. If you want the server to pass metadata results back in a
 different character set, use the `SET NAMES' statement to force the
 server to perform character set conversion. `SET NAMES' sets the
 `character_set_results' and other related system variables. (See 
 charset-connection.) Alternatively, a client program can perform the
 conversion after receiving the result from the server. It is more
 efficient for the client perform the conversion, but this option is not
 always available for all clients.
 
 If `character_set_results' 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').
 
 Error messages returned from the server to the client are converted to
 the client character set automatically, as with metadata.
 
 If you are using (for example) the `USER()' function for comparison or
 assignment within a single statement, don't worry. MySQL performs some
 automatic conversion for you.
 
      SELECT * FROM Table1 WHERE USER() = latin1_column;
 
 This works because the contents of `latin1_column' are automatically
 converted to UTF-8 before the comparison.
 
      INSERT INTO Table1 (latin1_column) SELECT USER();
 
 This works 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.  Because 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.
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