12.7 Column Character Set Conversion

To convert a binary or nonbinary string column to use a particular character set, use ALTER TABLE. For successful conversion to occur, one of the following conditions must apply:

• If the column has a binary data type (BINARY, VARBINARY, BLOB), all the values that it contains must be encoded using a single character set (the character set you're converting the column to). If you use a binary column to store information in multiple character sets, MySQL has no way to know which values use which character set and cannot convert the data properly.

• If the column has a nonbinary data type (CHAR, VARCHAR, TEXT), its contents should be encoded in the column character set, not some other character set. If the contents are encoded in a different character set, you can convert the column to use a binary data type first, and then to a nonbinary column with the desired character set.

Suppose that a table t has a binary column named col1 defined as VARBINARY(50). Assuming that the information in the column is encoded using a single character set, you can convert it to a nonbinary column that has that character set. For example, if col1 contains binary data representing characters in the greek character set, you can convert it as follows:

ALTER TABLE t MODIFY col1 VARCHAR(50) CHARACTER SET greek;

If your original column has a type of BINARY(50), you could convert it to CHAR(50), but the resulting values are padded with 0x00 bytes at the end, which may be undesirable. To remove these bytes, use the TRIM() function:

UPDATE t SET col1 = TRIM(TRAILING 0x00 FROM col1);

Suppose that table t has a nonbinary column named col1 defined as CHAR(50) CHARACTER SET latin1 but you want to convert it to use utf8mb4 so that you can store values from many languages. The following statement accomplishes this:

ALTER TABLE t MODIFY col1 CHAR(50) CHARACTER SET utf8mb4;

Conversion may be lossy if the column contains characters that are not in both character sets.

A special case occurs if you have old tables from before MySQL 4.1 where a nonbinary column contains values that actually are encoded in a character set different from the server's default character set. For example, an application might have stored sjis values in a column, even though MySQL's default character set was different. It is possible to convert the column to use the proper character set but an additional step is required. Suppose that the server's default character set was latin1 and col1 is defined as CHAR(50) but its contents are sjis values. The first step is to convert the column to a binary data type, which removes the existing character set information without performing any character conversion:

ALTER TABLE t MODIFY col1 BLOB;

The next step is to convert the column to a nonbinary data type with the proper character set:

ALTER TABLE t MODIFY col1 CHAR(50) CHARACTER SET sjis;

This procedure requires that the table not have been modified already with statements such as INSERT or UPDATE after an upgrade to MySQL 4.1 or higher. In that case, MySQL would store new values in the column using latin1, and the column would contain a mix of sjis and latin1 values and cannot be converted properly.

If you specified attributes when creating a column initially, you should also specify them when altering the table with ALTER TABLE. For example, if you specified NOT NULL and an explicit DEFAULT value, you should also provide them in the ALTER TABLE statement. Otherwise, the resulting column definition does not include those attributes.

To convert all character columns in a table, the ALTER TABLE ... CONVERT TO CHARACTER SET charset statement may be useful. See Section 15.1.9, “ALTER TABLE Statement”.