vi /etc/my.cnf

[mysqld]
character-set-server = utf8mb4
collation-server = utf8mb4_general_ci
skip-character-set-client-handshake
#忽略应用连接自己设置的字符编码，保持与全局设置一致
[client]
default-character-set=utf8mb4
[mysql]
default-character-set=utf8mb4

# 设置数据库字符集编码
ALTER DATABASE `dbname` CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

# 示例
mysql> show create database testdb;
+----------+-----------------------------------------------------------------+
| Database | Create Database             |
+----------+-----------------------------------------------------------------+
| testdb | CREATE DATABASE `testdb` /*!40100 DEFAULT CHARACTER SET utf8 */ |
+----------+-----------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> alter database `testdb` CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;
Query OK, 1 row affected (0.01 sec)

mysql> show create database testdb;
+----------+--------------------------------------------------------------------+
| Database | Create Database             |
+----------+--------------------------------------------------------------------+
| testdb | CREATE DATABASE `testdb` /*!40100 DEFAULT CHARACTER SET utf8mb4 */ |
+----------+--------------------------------------------------------------------+
1 row in set (0.00 sec)

# 修改表字符集
alter table `tb_name` default character set utf8mb4;

# 修改某字段字符集
alter table `tb_name` modify col_name varchar(20) character set utf8mb4;

# 同时修改表及字段字符集
alter table `tb_name` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;

# 如果某个库里面表比较多 可以拼接出要执行的批量修改语句
SELECT
 CONCAT( 'ALTER TABLE ', TABLE_NAME, ' CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci;' ) 
FROM
 information_schema.`TABLES`
WHERE
 TABLE_SCHEMA = 'testdb';