MySQL

MySQL is a popular, free software relational database management system. It is often used in conjunction with web applications (such as many PHP sites) but has gained many more enterprise-level features since its start in 1994.

Installing MySQL
Like any other software, the installation of MySQL in Gentoo is pretty straight-forward:

To have the database(s) started automatically at boot, add it to the default runlevel:

When the database is configured, you can start the service:

Preliminary configuration
The MySQL package maintainers (in Gentoo) will help you set up MySQL through the configuration option in the ebuild.

The configuration will create a database, set proper permissions on it and assist you in creating a good root password (this is for the MySQL root account, which is not related to the Linux root account).

In-database configuration
When the database is up and running, you can connect to it using the mysql client application.

Once connected, you can launch queries as well as administrative commands.

Resources
More information about MySQL and using MySQL can be found on
 * Official MySQL Community Edition documentation