Drupal additional database setup & connect example

<?php

use Drupal\Core\Database\Database;

// Method one.
$result = \Drupal::database()->query('SELECT * FROM my_additional_db.my_table')->fetchAllAssoc('my_column');
dump($result);

// Method two.
$conn = Database::setActiveConnection('my_extra_connection');
$conn = Database::getConnection();

// Execute any query against your additional database.
$table_names = $conn->schema()->findTables('%');
dump($table_names);
$result = \Drupal::database()->query('SELECT * FROM my_table')->fetchAllAssoc('my_column');
dump($result);

settings.php

<?php
// ...

// Append additional database info to the Drupal settings.php file.
$databases['extra_connection']['default'] = [
  'driver' => 'mysql',
  'database' => 'my_additional_db',
  'username' => 'my_additional_db_USER',
  'password' => 'my_additional_db_USER_PASSWORD',
  'host' => 'my_additional_db_host', // Might be a localhost or external host.
  'port' => '3306', // For the most db hosts the default 3306 port should work.
];