How to inject database connection in Drupal 8

Database connection can be injected in class object using dependency injection.

We can also use service method

\Drupal::database() 

This way of using database is suitable for XXX.module file but in class we can use dependency injection.

Using dependency injection :-

Drupal\Core\Database\Connection class for database connection.

use Drupal\Core\Database\Connection;
use Symfony\Component\DependencyInjection\ContainerInterface;

class CustomClass {

  /**
   * The database connection to be used.
   *
   * @var \Drupal\Core\Database\Connection
   */
  protected $database;

  /**
   * @param \Drupal\Core\Database\Connection $database
   *   The database connection to be used.
   */
  public function __construct(Connection $database) {
    $this->database = $database;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    $container->get('database')
  }

}

$container->get('database') here database is the service.

How to use it

For insert query :-

$query = $this->database->insert('custom_table')
        ->fields(['id', 'message'])
        ->values([1, 'some text']);
$result = $query->execute();

For checking table exists :-

$database_schema = $this->database->schema();
if (!($database_schema->tableExists('custom_table'))) {
  return FALSE;
}