1.问题原因
通过相关问题查阅，发现是由于navicat版本的问题造成连接失败。mysql8 之前的版本中加密规则是mysql_native_password,而在mysql8之后,加密规则是caching_sha2_password
2.解决
select version(); 查询版本号 看看是不是

2.输入命令修改加密规则

ALTER USER 'root'@'localhost' IDENTIFIED BY '12345678' PASSWORD EXPIRE NEVER;
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY '123456';
FLUSH PRIVILEGES; 

mysql> use mysql;  //先使用命令 use mysql
Database changed
mysql> select user,host,plugin from user where user='root'; // 在输入该命令
+------+-----------+-----------------------+ 
| user | host      | plugin                |
+------+-----------+-----------------------+
| root | localhost | mysql_native_password |
+------+-----------+-----------------------+
1 row in set (0.00 sec)

借鉴 https://blog.csdn.net/weixin_43111077/article/details/108811949