ubuntu 20.04系统自带源直接安装是MySQL 8.0，我要安装MySQL 5.7的版本的所以先进行换源

 在操作前先备份原来的sorce文件

sudo cp /etc/apt/sources.list /etc/apt/sources.list.old

修改sources.list文件，删除所有的配制信息，改成选择的是清华镜像源，保存退出。

sudo vim /etc/apt/sources.list

清华镜像源

# 清华镜像源
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse
deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse

更新镜像源和软件

# 更新镜像源

sudo apt-get update

# 更新软件

sudo apt-get upgrade

 apt-get install 方式安装mysql 5.7

sudo apt-get install mysql-server-5.7

遇到图片如下，两次输入数据库密码 下一步就安装完成。