When I tried to start mariadb, I got:

[root]# systemctl start mariadb

Failed to start mariadb.service: Unit not found.

And I also got this message:

Package 1:mariadb-5.5.56-2.el7.x86_64 is obsoleted by mysql-community-client-5.6.39-2.el7.x86_64 which is already installed

What should I do next?

Thank you all, Denny