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10.9. Column Character Set Conversion

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

Suppose that a table t has a binary column named col1 defined as BINARY(50). Assuming that the information in the column is encoded using a single character set, you can convert it to a non-binary 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 CHAR(50) CHARACTER SET greek;

Suppose that table t has a non-binary column named col1 defined as CHAR(50) CHARACTER SET latin1 but you want to convert it to use utf8 so that you can store values from many languages. The following statement accomplishes this:

ALTER TABLE t MODIFY col1 CHAR(50) CHARACTER SET utf8;

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 MySQL 4.0 or earlier where a non-binary 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 latin1. 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 BINARY(50);

The next step is to convert the column to a non-binary 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 later. In that case, MySQL would store new values in the column using latin1, and the column will 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 will not include those attributes.

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