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,
user names, 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 statements
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(),
SESSION_USER(),
SYSTEM_USER(),
DATABASE(), and
VERSION() functions have the
UTF-8 character set by default.
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
Section 10.1.4, “Connection Character Sets and Collations”.) Alternatively, a client
program can perform the conversion after receiving the result
from the server. It is more efficient for the client to
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 t1 WHERE USER() = latin1_column;
This works because the contents of
latin1_column are automatically converted
to UTF-8 before the comparison.
INSERT INTO t1 (latin1_column) SELECT USER();
This works because the contents of
USER() are automatically
converted to latin1 before the assignment.
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. For more information about coercion of strings, see Section 10.1.8.4, “Collation of Expressions”.