参考：cubuntu安装mysql5.6_mob649e81553a70的技术博客_51CTO博客
问题1：sudo apt install mysql-server-5.7 -y 若提示mysql-server 没有可安装候选
答：
sudo nano /etc/apt/sources.list
在开头加入：
阿里源
deb http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-security main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-updates main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-proposed main restricted universe multiverse
deb-src http://mirrors.aliyun.com/ubuntu/ bionic-backports main restricted universe multiverse
更新后即可安装：
sudo apt-get update
问题2：安装完成后，mysql -u root -p命令提示Access denied for user 'root'@'localhost'
强行登录
sudo mysql
问题3：如何设置mysql的开机启动
sudo systemctl enable mysql