<?php
  // Database service. It's better to use dependency injection in your class.
  $database = \Drupal::service('database');
  $earth_radius = 3959;
  // Coordinates of the place from which you would like to get nearest places.
  $lat = '52.486081';
  $lon = '13.404616';
  // How many nearest places we would like to show?
  $limit = 10;
  // Radius to search.
  $radius = 100;
  $query = $database->select('my_table_name_from_db', 'destination');
  $query->fields('destination');
  $query->orderBy('distance');
  $query->range(0, $limit);
  // field_latitude and field_longitude - columns names from 'my_table_name_from_db'
  // where coordinates stored.
  $expr = "(
    $earth_radius * acos(
      cos(radians(:lat))
      * cos(radians(field_latitude))
      * cos(radians(field_longitude) - radians(:lon))
      + sin(radians(:lat))
      * sin(radians(field_latitude))
    )
  )";
  $query->addExpression($expr, 'distance', array(':lat' => $lat, ':lon' => $lon));
  $query->having('distance <= :radius', array(':radius' => $radius));
  $destinations = $query->execute()->fetchAll();

<?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')
    );
  }
}

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

$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();

<?php
/**
 * @file
 * Contains \Drupal\our_custom_module_name\Plugin\Validation\Constraint\ExistsEntityWithNameConstraint.
 */
namespace Drupal\our_custom_module_name\Plugin\Validation\Constraint;
use Symfony\Component\Validator\Constraint;
/**
 * Constraint for checking if a default entity already exists.
 *
 * @Constraint(
 * id = "ExistsEntityWithName",
 * label = @Translation("Entity exists", context = "Validation"),
 * type = { "entity" }
 * )
 */
class ExistsEntityWithNameConstraint extends Constraint {
 /**
 * Message shown when the entity already exists.
 *
 * @var string
 */
 public $messageExists = 'Entity %entity already exists';
}

<?php
/**
 * @file
 * Contains \Drupal\our_custom_module_name\Plugin\Validation\Constraint\ExistsEntityWithNameConstraintValidator.
 */
namespace Drupal\our_custom_module_name\Plugin\Validation\Constraint;
use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
use Drupal\Core\Entity\EntityTypeManager;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;
/**
 * Validates the ExistsEntityWithNameConstraint.
 */
class ExistsEntityWithNameConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {
  /**
   * Validator 2.5 and upwards compatible execution context.
   *
   * @var \Symfony\Component\Validator\Context\ExecutionContextInterface
   */
  protected $context;
  /**
   * @var \Drupal\Core\Entity\EntityTypeManager
   */
  protected $entityTypeManager;
  /**
   * Constructs a new ExistsEntityWithNameConstraintValidator.
   *
   * @param \Drupal\Core\Entity\EntityTypeManager
   *   The user storage handler.
   */
  public function __construct(EntityTypeManager $entity_type_manager) {
    $this->entityTypeManager = $entity_type_manager;
  }
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static($container->get('entity_type.manager'));
  }
  /**
   * {@inheritdoc}
   *
   * Here we validate entities and check if the same entity
   * (with the same name) already exists.
   */
  public function validate($entity, Constraint $constraint) {
    $name = $entity->name->value;
    if (isset($name)) {
      $entity_storage = $this->entityTypeManager->getStorage($entity->bundle());
      // Check if we already have the same entity.
      $ex_entity = $entity_storage->loadByProperties(array('name'=> $name));
      if (!empty($ex_entity) && is_array($ex_entity)) {
        // Takes entity id.
        $ex_entity_id = array_shift($ex_entity)->id();
        // If we just update the same entity, then skip validation.
        if ($entity->id() != $ex_entity_id) {
          $this->context->buildViolation($constraint->messageExists, array(
            '%entity' => $name
          ))
            ->atPath('name')
            ->addViolation();
        }
      }
    }
  }
}

...
constraints = {
 "ExistsEntityWithName" = {}
}
...