在网上找了很久，都是到官网下载安装包，最后终于找到了一个非常简便的方法！

⭐参考文章：ubuntu 20.04安装mysql5.7 - wltianOne - 博客园

查看自己源对应的mysql版本：

apt-cache policy mysql-server

说明直接使用apt-get install mysql-server会直接安装mysql8.0版本 

root@ruihanna-virtual-machine:/etc/apt# apt-cache policy mysql-server
mysql-server:
  已安装：(无)
  候选： 8.0.39-0ubuntu0.22.04.1
  版本列表：
     8.0.39-0ubuntu0.22.04.1 500
        500 http://mirrors.aliyun.com/ubuntu jammy-updates/main amd64 Packages
        500 http://mirrors.aliyun.com/ubuntu jammy-updates/main i386 Packages
        500 http://mirrors.aliyun.com/ubuntu jammy-security/main amd64 Packages
        500 http://mirrors.aliyun.com/ubuntu jammy-security/main i386 Packages
     8.0.28-0ubuntu4 500
        500 http://mirrors.aliyun.com/ubuntu jammy/main amd64 Packages
        500 http://mirrors.aliyun.com/ubuntu jammy/main i386 Packages

 换源实现安装mysql5.7：

#  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

备份sources.list， 并换为以上阿里源

cp /etc/apt/sources.list /etc/apt/sources.list.back
cd /etc/apt
vim sources.list # 换源

# 更新源
gpg --keyserver keyserver.ubuntu.com --recv 3B4FE6ACC0B21F32
gpg --export --armor 3B4FE6ACC0B21F32 | apt-key add -
apt-get update

 再次查看对应mysql版本：

root@ruihanna-virtual-machine:/home/hadoop# apt-cache policy mysql-server
mysql-server:
  已安装：(无)
  候选： 5.7.42-0ubuntu0.18.04.1
  版本列表：
     5.7.42-0ubuntu0.18.04.1 500
        500 http://mirrors.aliyun.com/ubuntu bionic-security/main amd64 Packages
        500 http://mirrors.aliyun.com/ubuntu bionic-security/main i386 Packages
        500 http://mirrors.aliyun.com/ubuntu bionic-updates/main amd64 Packages
        500 http://mirrors.aliyun.com/ubuntu bionic-updates/main i386 Packages
     5.7.21-1ubuntu1 500
        500 http://mirrors.aliyun.com/ubuntu bionic/main amd64 Packages
        500 http://mirrors.aliyun.com/ubuntu bionic/main i386 Packages

安装mysql5.7，查看mysql版本，验证是否安装成功

apt-get -y install mysql-server-5.7 # 安装
mysql -V # 查看mysql版本

最后，换回镜像源

cd /etc/apt
mv sources.list sources.list.old
mv sources.list.back  sources.list
apt-get update