How to update column with null value

$fields=array();
foreach ($fields as $key => $value) {
        if (trim($value) == '' && !is_null($value) && !is_bool($value)){
            $fields[$key] = null;
        }
    }


$sql = 'SELECT * FROM table_name WHERE column IS NULL';

$sql = 'SELECT * FROM table_name WHERE column IS NOT NULL';