浪费了一下午时间（可惜啊！！！！）
初始化mysql会给你一个随机密码，记录这个密码，如果之前电脑有安装过mysql并且没有初始化过，这个密码是登陆不了的；
这时候要先卸载mysql，再mysql -install 一下（不行就多卸载再安装几次）。
然后修改root密码，要用下面的方式修改：
Microsoft Windows [版本 10.0.17134.285]
(c) 2018 Microsoft Corporation。保留所有权利。
C:\WINDOWS\system32>cd C:\mysql8\mysql-8.0.12-winx64\bin
C:\mysql8\mysql-8.0.12-winx64\bin>mysql -u root -p
Enter password: ************
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 13
Server version: 8.0.12
Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> select host,user,authentication_string from mysql.user;
ERROR 1820 (HY000): You must reset your password using ALTER USER statement before executing this statement.
mysql> alter user 'root'@'localhost' identified by 'youpassword';
Query OK, 0 rows affected (0.21 sec)
mysql> alter user 'root'@'localhost' identified by '123456';
Query OK, 0 rows affected (0.08 sec)
mysql> flush privileges;
Query OK, 0 rows affected (0.03 sec)
mysql>