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
SELECTstatements for tables in
INFORMATION_SCHEMAwould 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
VERSION() functions have the
UTF-8 character set by default.
The server sets the
variable to the name of the metadata character set:
mysql> SHOW VARIABLES LIKE 'character_set_system'; +----------------------+---------+ | Variable_name | Value | +----------------------+---------+ | character_set_system | utf8mb3 | +----------------------+---------+
Storage of metadata using Unicode does not
mean that the server returns headers of columns and the results
DESCRIBE functions in the
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
variable, which has a default value of
utf8mb4. 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 10.4, “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
character_set_results is set
NULL, no conversion is performed and the
server returns metadata using its original character set (the
set indicated by
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
latin1 before the assignment.
Although automatic conversion is not in the SQL standard, the standard 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 10.8.4, “Collation Coercibility in Expressions”.