SQL 'select' query where parameter is NULL

Hi,

is it possible to run a SELECT query where the parameter is NULL?
I am trying the following but it does not work for me:

$connection=Yii::app()->db;
		
$sql = "SELECT id, categoryName FROM categories WHERE parentId=:parentId";
		
$command=$connection->createCommand($sql);

$parentId = NULL;		

$command->bindParam(":parentId",$parentId,PDO::PARAM_INT);
		
print_r($command->queryAll());

I also tried the following without any luck:

$command->bindParam(":parentId",$parentId,PDO::PARAM_NULL);