In the latest release of CentOS, you will find that MariaDB has replaced MySQL. It is correct to say that there are similarities between the two database systems, but it is also important to recognize that they are both very different systems. Based on this premise, a typical issue often begins at the point of installation, where the following message may prove to be more than uncomfortable:

Redirecting to /bin/systemctl start  mysqld.service. 
Failed to issue method call: Unit mysqld.service failed to load: No such file or directory.

In this case, your first action is to verify what is currently installed. This can be achieved by running one or more of the following commands:

# which mysql
# ls -la /bin/my*
# ps -ef | grep mysql

However, given that the output will seem to be misleading to the untrained eye, this issue provides a point of confusion, and it happens as a result of trying to run the MySQL installation and startup commands. Yes, it is an easy...