1,在Centos7安装MySQL完毕后，提示如下：

[root@VM_176_159_centos tmp]# yum install mysql 
Loaded plugins: fastestmirror, langpacks
Loading mirror speeds from cached hostfile
Package 1:mariadb-5.5.44-2.el7.centos.x86_64 already installed and latest version
Nothing to do
[root@VM_176_159_centos tmp]# 

显示的是mariadb，原来Centos7已经将MySQL替换成MariaDB，它是MySQL的一个分支，完全兼容MySQL的功能。

2,尝试启动MySQL，结果报错了

[root@VM_176_159_centos tmp]# service mysql start
Redirecting to /bin/systemctl start  mysql.service
Failed to issue method call: Unit mysql.service failed to load: No such file or directory.

CentOS7需要使用systemctl命令，对于MariaDB用 systemctl start|stop|restart mariadb.service；对于MySQL就用 systemctl start|stop|restart mysql.service