Every “character” column (that is, a column of
type CHAR,
VARCHAR, or
TEXT) has a column character
set and a column collation. Column definition syntax for
CREATE TABLE and
ALTER TABLE has optional
clauses for specifying the column character set and collation:
col_name{CHAR | VARCHAR | TEXT} (col_length) [CHARACTER SETcharset_name] [COLLATEcollation_name]
These clauses can also be used for
ENUM and
SET columns:
col_name{ENUM | SET} (val_list) [CHARACTER SETcharset_name] [COLLATEcollation_name]
Examples:
CREATE TABLE t1
(
col1 VARCHAR(5)
CHARACTER SET latin1
COLLATE latin1_german1_ci
);
ALTER TABLE t1 MODIFY
col1 VARCHAR(5)
CHARACTER SET latin1
COLLATE latin1_swedish_ci;
MySQL chooses the column character set and collation in the following manner:
If both CHARACTER SET
and
charset_nameCOLLATE
are
specified, character set
collation_namecharset_name and collation
collation_name are used.
CREATE TABLE t1
(
col1 CHAR(10) CHARACTER SET utf8 COLLATE utf8_unicode_ci
) CHARACTER SET latin1 COLLATE latin1_bin;
The character set and collation are specified for the
column, so they are used. The column has character set
utf8 and collation
utf8_unicode_ci.
If CHARACTER SET
is
specified without charset_nameCOLLATE, character
set charset_name and its
default collation are used.
CREATE TABLE t1
(
col1 CHAR(10) CHARACTER SET utf8
) CHARACTER SET latin1 COLLATE latin1_bin;
The character set is specified for the column, but the
collation is not. The column has character set
utf8 and the default collation for
utf8, which is
utf8_general_ci. To see the default
collation for each character set, use the
SHOW CHARACTER SET
statement.
If COLLATE
is
specified without collation_nameCHARACTER SET, the
character set associated with
collation_name and collation
collation_name are used.
CREATE TABLE t1
(
col1 CHAR(10) COLLATE utf8_polish_ci
) CHARACTER SET latin1 COLLATE latin1_bin;
The collation is specified for the column, but the
character set is not. The column has collation
utf8_polish_ci and the character set is
the one associated with the collation, which is
utf8.
Otherwise (neither CHARACTER SET nor
COLLATE is specified), the table
character set and collation are used.
CREATE TABLE t1
(
col1 CHAR(10)
) CHARACTER SET latin1 COLLATE latin1_bin;
Neither the character set nor collation is specified for
the column, so the table defaults are used. The column has
character set latin1 and collation
latin1_bin.
The CHARACTER SET and
COLLATE clauses are standard SQL.
If you use ALTER TABLE to
convert a column from one character set to another, MySQL
attempts to map the data values, but if the character sets are
incompatible, there may be data loss.