官网文档:https://dev.mysql.com/doc/refman/5.7/en/alter-table.html
# 仅改变表定义的字符集和排序规则
alter table table_name character set utf8mb4 collate utf8mb4_general_ci;
# 改变表定义和字符类型字段的字符集和排序规则
alter table table_name convert to character set utf8mb4 collate utf8mb4_general_ci;
具体操作注意事项可以看官方文档