將表的所有資料轉為另外一種字符集和校對規則

To change the table default character set and all character columns (CHAR, VARCHAR, TEXT) to a new character set, use a statement like this:

ALTER TABLE tbl_name
CONVERT TO CHARACTER SET charset_name [COLLATE collation_name];

The statement also changes the collation of all character columns. If you specify no COLLATE

 clause to indicate which collation to use, the statement uses default collation for the character set. If this collation is inappropriate for the intended table use (for example, if it would change from a case-sensitive collation to a case-insensitive collation), specify a collation explicitly.

For a column that has a data type of 

VARCHAR or one of the TEXT types, CONVERT TO CHARACTER SET will change the data type as necessary to ensure that the new column is long enough to store as many characters as the original column. For example, a TEXT column has two length bytes, which store the byte-length of values in the column, up to a maximum of 65,535. For a latin1TEXT column, each character requires a single byte, so the column can store up to 65,535 characters. If the column is converted to utf8, each character might require up to three bytes, for a maximum possible length of 3 × 65,535 = 196,605 bytes. That length will not fit in a TEXT column's length bytes, so MySQL will convert the data type to MEDIUMTEXT, which is the smallest string type for which the length bytes can record a value of 196,605. Similarly, a VARCHAR column might be converted to MEDIUMTEXT.

注意：

使用convert to character set charset_name,如果轉換後的資料型別不能儲存全部資料，會發生資料型別變化。

比如text最多可以儲存65535個位元組，latin1字符集下，一個字元佔用一個位元組，所以也就是65535個字元，轉換為utf8，一個字元至多可以佔用3個位元組，所以，最壞的情況，轉換後就需要65535*3個位元組，超出text容量，Mysql會自動將資料型別轉為mediumtext

修改某列的字符集

To avoid data type changes of the type just described, do not use CONVERT TO CHARACTER SET. Instead, use MODIFY to change individual columns. For example:

ALTER TABLE t MODIFY latin1_text_col TEXT CHARACTER SET utf8;
ALTER TABLE t MODIFY latin1_varchar_col VARCHAR(M) CHARACTER SET utf8;

If you specify CONVERT TO CHARACTER SET binary, the CHAR, VARCHAR, and TEXT columns are converted to their corresponding binary string types (BINARY, VARBINARY, BLOB). This means that the columns no longer will have a character set and a subsequent CONVERT TO operation will not apply to them.

If charset_name is DEFAULT, the database character set is used.

修改某表的預設字符集和校對規則

只會對以後的列有影響，對已經建立的列，不會產生影響。

To change only the default character set for a table, use this statement:

ALTER TABLE tbl_name DEFAULT CHARACTER SET charset_name [COLLATE collation_name]; 參考