步骤如下:
1.获取mysql-apt-config 工具
wget https://dev.mysql.com/get/mysql-apt-config_0.8.11-1_all.deb
2.安装config 工具 第一项点进去选择5.7（默认是8.0）
dpkg -i mysql-apt-config_0.8.11-1_all.deb
3.更新源
sudo apt-get update
4.安装mysqk
sudo apt-get install mysql-server
5.理论上讲，上一步过程中可能会出现输入密码，但是这里说下没有出现密码的选项的解决方案【没有产生初始化密码 /vsr/log/mysqld.log】
终端输入： sudo cat /etc/mysql/debian.cnf
显示内容：
# Automatically generated for Debian scripts. DO NOT TOUCH!
[client]
host = localhost
user = debian-sys-maint
password = PGBiNPneSGXRlayQ
socket = /var/run/mysqld/mysqld.sock
[mysql_upgrade]
host = localhost
user = debian-sys-maint
password = PGBiNPneSGXRlayQ
socket = /var/run/mysqld/mysqld.sock
其中有user和password可以用来登录，登录后再修改密码；
终端输入：mysql -u debian-sys-maint -p ，再输入密码后，进入ｍｙｓｑｌ中
elcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.24-0ubuntu0.18.04.1 (Ubuntu)
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>
在mysql客户端中输入依次输入以下内容：
mysql> show databases;
mysql> use mysql;
mysql> update user set authentication_string=PASSWORD("root") where user="root";
mysql> update user set plugin="mysql_native_password";
mysql> flush privileges;
mysql> quit;
其中PASSWORD后面的root可以为其他自定义的密码，
在终端中输入 sudo /etc/init.d/mysql restart; 命令，重启ｍｙｓｑｌ服务
使用新密码登录mysql：mysql -u root -p
