mysql 修改表、列的字符集和校对规则详解数据库

将表的所有数据转为另外一种字符集和校对规则

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 latin1 TEXT 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,如果转换后的数据类型不能存储全部数据，会发生数据类型变化。

比如text最多可以存储65535个字节，latin1字符集下，一个字符占用一个字节，所以也就是65535个字符，转换为utf8，一个字符至多可以占用3个字节，所以，最坏的情况，转换后就需要65535*3个字节，超出text容量，Mysql会自动将数据类型转为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.

修改某表的默认字符集和校对规则

只会对以后的列有影响，对已经创建的列，不会产生影响。

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];

参考

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