使用ALTER DATABASE和ALTER TABLE命令。

ALTER DATABASE databasename CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE tablename CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

或者如果你仍然使用MySQL 5.5.2或更早的版本，不支持4字节UTF-8，使用utf8而不是utf8mb4:

ALTER DATABASE databasename CHARACTER SET utf8 COLLATE utf8_unicode_ci;
ALTER TABLE tablename CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci;

如果数据不在相同的字符集中，您可以考虑http://dev.mysql.com/doc/refman/5.0/en/charset-conversion.html中的这个片段

如果列具有非二进制数据类型(CHAR、VARCHAR、TEXT)，则其 内容应该在列字符集中编码，而不是其他字符集 字符集。如果内容用不同的字符编码 设置后，您可以先将列转换为使用二进制数据类型，然后 然后转换到具有所需字符集的非二进制列。

这里有一个例子:

 ALTER TABLE t1 CHANGE c1 c1 BLOB;
 ALTER TABLE t1 CHANGE c1 c1 VARCHAR(100) CHARACTER SET utf8;

确保选择正确的排序规则，否则可能会得到唯一的键冲突。如。 Éleanore和Eleanore在某些排序中可能被认为是相同的。

旁白:

我曾遇到过这样的情况，电子邮件中的某些字符“坏了”，尽管它们在数据库中以UTF-8格式存储。如果您使用utf8数据发送电子邮件，您可能还需要将电子邮件转换为utf8发送。

在phpailer中，只需更新这一行:public $CharSet = 'utf-8';

要更改数据库本身的字符集编码为UTF-8，在mysql>提示符处键入以下命令。将DBNAME替换为数据库名称。

ALTER DATABASE DBNAME CHARACTER SET utf8 COLLATE utf8_general_ci;

你也可以使用数据库工具Navicat，这样做更容易。

湿 婆。

右键单击您的数据库，并在下拉菜单中选择数据库属性和更改

utf8到utf8mb4:

1.显示所有数据库默认字符集:

SELECT SCHEMA_NAME 'YOUR_DATABASE_NAME', 
default_character_set_name 'charset', 
DEFAULT_COLLATION_NAME 'collation' 
FROM information_schema.SCHEMATA;

2.显示所有表的状态(字符集)，关注列的“collation”:

use YOUR_DATABASE_NAME;    
SHOW TABLE STATUS ;

3.生成转换sql: 转换数据库和所有表utf8mb4,utf8mb4_unicode_ci

USE information_schema;
SELECT CONCAT("ALTER DATABASE `",table_schema,"` CHARACTER SET = utf8mb4 COLLATE = utf8mb4_unicode_ci;") AS _sql
FROM `TABLES` WHERE table_schema LIKE "YOUR_DATABASE_NAME" AND TABLE_TYPE='BASE TABLE' GROUP BY table_schema UNION
SELECT CONCAT("ALTER TABLE `",table_schema,"`.`",table_name,"` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;") AS _sql  
FROM `TABLES` WHERE table_schema LIKE "YOUR_DATABASE_NAME" AND TABLE_TYPE='BASE TABLE' GROUP BY table_schema, TABLE_NAME 
/*include all columns, commonly don't need this.*/
/*
UNION
SELECT CONCAT("ALTER TABLE `",`COLUMNS`.table_schema,"`.`",`COLUMNS`.table_name, "` CHANGE `",column_name,"` `",column_name,"` ",data_type,"(",character_maximum_length,") CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci",IF(is_nullable="YES"," NULL"," NOT NULL"),";") AS _sql 
FROM `COLUMNS` INNER JOIN `TABLES` ON `TABLES`.table_name = `COLUMNS`.table_name WHERE `COLUMNS`.table_schema like "YOUR_DATABASE_NAME" and data_type in ('varchar','char') AND TABLE_TYPE='BASE TABLE' UNION
SELECT CONCAT("ALTER TABLE `",`COLUMNS`.table_schema,"`.`",`COLUMNS`.table_name, "` CHANGE `",column_name,"` `",column_name,"` ",data_type," CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci",IF(is_nullable="YES"," NULL"," NOT NULL"),";") AS _sql 
FROM `COLUMNS` INNER JOIN `TABLES` ON `TABLES`.table_name = `COLUMNS`.table_name WHERE `COLUMNS`.table_schema like "YOUR_DATABASE_NAME" and data_type in ('text','tinytext','mediumtext','longtext') AND TABLE_TYPE='BASE TABLE';
*/

4.运行生成的SQL语句。

5.刷新数据库。

6.检查:

SHOW TABLE STATUS ;

使用ALTER DATABASE和ALTER TABLE命令。

ALTER DATABASE databasename CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE tablename CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

或者如果你仍然使用MySQL 5.5.2或更早的版本，不支持4字节UTF-8，使用utf8而不是utf8mb4:

ALTER DATABASE databasename CHARACTER SET utf8 COLLATE utf8_unicode_ci;
ALTER TABLE tablename CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci;