protected function getLeftComparison(SearchCriterion $searchCriterion)
    {
        switch ($searchCriterion->getComparison()) {
            case SearchCriterion::GREATER_EQUAL:
            case SearchCriterion::GREATER_THAN:
                return $searchCriterion->getValue();
            case SearchCriterion::ISNULL:
            case SearchCriterion::ISNOTNULL:
            case SearchCriterion::LESS_EQUAL:
            case SearchCriterion::LESS_THAN:
                return '*';
            default:
                throw new InvalidArgumentException('Invalid comparison for RangeCriterion');
        }
    }