MySQL :: MySQL 8.4 Reference Manual :: 12.3.5 Column Character Set and Collation

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MySQL 8.4 Reference Manual / ... / Column Character Set and Collation

12.3.5 列字符集和排序规则

Every “character” column (that is, a column of type CHAR, VARCHAR, a TEXT type, or any synonym) 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 SET charset_name]
    [COLLATE collation_name]

These clauses can also be used for ENUM and SET columns:

col_name {ENUM | SET} (val_list)
    [CHARACTER SET charset_name]
    [COLLATE collation_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 charset_name and COLLATE collation_name are specified, character set charset_name and collation collation_name are used.
```
CREATE TABLE t1
(
    col1 CHAR(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_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 utf8mb4 and collation utf8mb4_unicode_ci.
If CHARACTER SET charset_name is specified without COLLATE, character set charset_name and its default collation are used.
```
CREATE TABLE t1
(
    col1 CHAR(10) CHARACTER SET utf8mb4
) CHARACTER SET latin1 COLLATE latin1_bin;
```
The character set is specified for the column, but the collation is not. The column has character set utf8mb4 and the default collation for utf8mb4, which is utf8mb4_0900_ai_ci. To see the default collation for each character set, use the SHOW CHARACTER SET statement or query the INFORMATION_SCHEMA CHARACTER_SETS table.
If COLLATE collation_name is specified without CHARACTER SET, the character set associated with collation_name and collation collation_name are used.
```
CREATE TABLE t1
(
    col1 CHAR(10) COLLATE utf8mb4_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 utf8mb4_polish_ci and the character set is the one associated with the collation, which is utf8mb4.
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.

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