public function onRelationalExpression($expressionType, $left, $right) {
        switch ($expressionType) {
            case ExpressionType::GREATERTHAN:
                return $this->_prepareBinaryExpression(self::GREATERTHAN, $left, $right);
                break;
            case ExpressionType::GREATERTHAN_OR_EQUAL:
                return $this->_prepareBinaryExpression(
                                self::GREATERTHAN_OR_EQUAL, $left, $right
                );
                break;
            case ExpressionType::LESSTHAN:
                return $this->_prepareBinaryExpression(self::LESSTHAN, $left, $right);
                break;
            case ExpressionType::LESSTHAN_OR_EQUAL:
                return $this->_prepareBinaryExpression(
                                self::LESSTHAN_OR_EQUAL, $left, $right
                );
                break;
            case ExpressionType::EQUAL:
                return $this->_prepareBinaryExpression(self::EQUAL, $left, $right);
                break;
            case ExpressionType::NOTEQUAL:
                return $this->_prepareBinaryExpression(self::NOTEQUAL, $left, $right);
                break;
            default:
                throw new \InvalidArgumentException('onArithmeticExpression');
        }
    }