Big changes in open source world, i was just installing zabbix server and mysql database is required for zabbix. After installing Centos 7 from minimal install iso i didn't get mysql by "yum install mysql-server so i came to know about changes that "MySQL is no longer in CentOS’s repositories and MariaDB has become the default database" who knows mariaDB ??
any way you can still install mysql by installing official mysql community Yum's Repo file for "yum install mysql-server"
Method : Download and install from this url
or simply rpm -ivh http://repo.mysql.com/mysql-community-release-el7-5.noarch.rpm