参考：https://www.linode.com/docs/databases/mysql/how-to-install-mysql-on-centos-7
centos 7上没有办法使用yum install mysql等命令了：
wget http://repo.mysql.com/mysql-community-release-el7-5.noarch.rpm
sudo rpm -ivh mysql-community-release-el7-.noarch.rpm
yum update
sudo yum install mysql-server
sudo systemctl start mysqld
完毕。