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<?php 

$query = \Drupal::entityQuery('node')
  ->condition('status', NODE_PUBLISHED)
  ->condition('type', 'custom_type');
$and = $query->andConditionGroup();
$and->condition('custom_taxonomy', 2);
$query->condition($and);
$and = $query->andConditionGroup();
$and->condition('custom_taxonomy', 8);
$query->condition($and);
$result = $query->execute();

->range(0, 2);

$query = \Drupal::entityQuery('test_entity');
        $query->notExists('field_date');
        $ids=$query->execute();
        
        
        
$thing_id = 101;
$q = \Drupal::entityQueryAggregate('node')
  ->condition('type', 'group')
  ->condition('field_things', $thing_id)
  ->groupBy('nid')
  ->conditionAggregate('field_things', 'COUNT', 1);
$res = $q->execute();


        
drill down        
dpm($entity->uid->entity->name->value);
dpm($entity->field_item_preview->entity->name);        



the oo way

$nids = \Drupal::entityTypeManager()
          ->getListBuilder('node')
          ->getStorage()
          ->loadByProperties([
            'type' => 'mytype',
            'status' => 1,
        ]);