MySQL and MariaDB are installed via the same services.mysql configuration.

MySQL is a widely used open source relational database management system (RDBMS) that offers various features, tools, and services for data warehousing, analytics, machine learning, and more.
MariaDB is a popular and stable fork of MySQL that is compatible with MySQL and has additional enhancements and features.

Setup MySQL

Setup and enable Mysql database daemon (in this example: latest stable version in nixpkgs)

services.mysql = {
  enable = true;
  package = pkgs.mysql;
};

Setup MariaDB

Setup and enable MariaDB database daemon (in this example: version 11.0)

services.mysql = {
  enable = true;
  package = pkgs.mariadb_110;
};

Maintenance

Upgrade

NixOS will not run mysql_upgrade automatically for you after upgrading to a new major version, because it is a "dangerous" operation (can lead to data corruption) and users are strongly advised (by MariaDB upstream) to backup their database before running mysql_upgrade.

mysqldump -u root -p --all-databases > alldb.sql

After backup is completed, you can proceed with the upgrade process

mysql_upgrade