每次想用yum安装旧版本的mysql时，发现都找不到mysql的yum安装源，在官网只能找到最新版本mysql的yum源。后来才知道，原来最新的mysql的yum源也是包含了旧版本的mysql的yum源，只需要开启改安装源即可。
系统版本：Centos7.7
MySQL版本：5.7
安装方法：使用官方Yum源安装
MySQL配置文件：/etc/my.cnf
具体步骤如下：
wget https://repo.mysql.com//mysql80-community-release-el7-3.noarch.rpm
rpm -Uvh mysql80-community-release-el7-3.noarch.rpm
sed -i '21c enabled=1' /etc/yum.repos.d/mysql-community.repo
sed -i '28c enabled=0' /etc/yum.repos.d/mysql-community.repo
yum clean all
yum install mysql-server mysql -y
systemctl start mysqld
cat /var/log/mysqld.log |grep pass
wget https://repo.mysql.com//mysql80-community-release-el7-3.noarch.rpm
rpm -Uvh mysql80-community-release-el7-3.noarch.rpm
sed -i '21c enabled=1' /etc/yum.repos.d/mysql-community.repo
sed -i '28c enabled=0' /etc/yum.repos.d/mysql-community.repo
yum clean all
yum install mysql-server mysql -y
systemctl start mysqld
cat /var/log/mysqld.log |grep pass