Work with database in Drupal 8

For creating database queries in Drupal 8, we should add dependency to ´Drupal\Core\Database\Connection´ class inside our class.

<?php
namespace Drupal\our_custom_module_name;

use Drupal\Core\Database\Connection;

/**
 * Our class, where we will communicate with database.
 */
class LetsTalkWithDatabase {
  /**
   * @var \Drupal\Core\Database\Connection
   */
  protected $database;

  /**
   * Construct.
   */
  public function __construct(Connection $database) {
    $this->database = $database;
  }

  /**
   * Create function.
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('database')
    );
  }
}

The Query with condition

$nodes = $this->database('node', 'n')
 ->fields('n', array('nid', 'title'))
 ->condition('n.type', 'page')
 ->condition('n.uid', 1)
 ->execute()
 ->fetchAll();

The Query with joining

$query = $this->database('node', 'n');
$query->innerJoin('users', 'u', 'n.uid = u.uid');
$query->fields('n', array('title'));
$query->fields('u', array('name'));
$nodes = $query->execute()->fetchAll();