MySQL默认的utf8只支持三字节字符，不支持Emoji表情符（四字节），如果有四字节的字符写入会报错。从MySQL 5.5开始，提供了utf8mb4，支持四字节的字符。

许多使用iphone的用户来在填写昵称的时候会加入Emoji表情符，如果MySQL没有使用utf8mb4字符集，这样的昵称信息就无法写入。

一、升级前。

mysql> SHOW VARIABLES LIKE 'character_set%';

+--------------------------+--------------------------------------------------------------------+
| Variable_name            | Value                                                              |
+--------------------------+--------------------------------------------------------------------+
| character_set_client     | utf8                                                               |
| character_set_connection | utf8                                                               |
| character_set_database   | utf8                                                               |
| character_set_filesystem | binary                                                             |
| character_set_results    | utf8                                                               |
| character_set_server     | utf8                                                               |
| character_set_system     | utf8                                                               |
| character_sets_dir       | /home/sdkserver/local/mysql-5.5.43-linux2.6-x86_64/share/charsets/ |
+--------------------------+--------------------------------------------------------------------+
8 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'collation%';

+----------------------+-----------------+
| Variable_name        | Value           |
+----------------------+-----------------+
| collation_connection | utf8_general_ci |
| collation_database   | utf8_unicode_ci |
| collation_server     | utf8_unicode_ci |
+----------------------+-----------------+
3 rows in set (0.00 sec)

二、修改配置文件。

打开MySQL的配置文件，将字符集的配置修改成如下：

[client]  
default-character-set = utf8mb4

[mysql]  
default-character-set = utf8mb4

[mysqld]  
character-set-client-handshake = FALSE
character-set-server = utf8mb4
collation-server = utf8mb4_general_ci
init_connect='SET NAMES utf8mb4'

三、升级后。

mysql> SHOW VARIABLES LIKE 'character_set%';

+--------------------------+--------------------------------------------------------------------+
| Variable_name            | Value                                                              |
+--------------------------+--------------------------------------------------------------------+
| character_set_client     | utf8mb4                                                            |
| character_set_connection | utf8mb4                                                            |
| character_set_database   | utf8mb4                                                            |
| character_set_filesystem | binary                                                             |
| character_set_results    | utf8mb4                                                            |
| character_set_server     | utf8mb4                                                            |
| character_set_system     | utf8                                                               |
| character_sets_dir       | /home/nieyong/local/mysql-5.5.43/share/charsets/                   |
+--------------------------+--------------------------------------------------------------------+
8 rows in set (0.00 sec)

mysql> SHOW VARIABLES LIKE 'collation%';

+----------------------+--------------------+
| Variable_name        | Value              |
+----------------------+--------------------+
| collation_connection | utf8mb4_general_ci |
| collation_database   | utf8mb4_general_ci |
| collation_server     | utf8mb4_general_ci |
+----------------------+--------------------+
3 rows in set (0.00 sec)