public function walkNullIfExpression($nullIfExpression)
    {
        $firstExpression = is_string($nullIfExpression->firstExpression)
            ? $this->conn->quote($nullIfExpression->firstExpression)
            : $this->walkSimpleArithmeticExpression($nullIfExpression->firstExpression);

        $secondExpression = is_string($nullIfExpression->secondExpression)
            ? $this->conn->quote($nullIfExpression->secondExpression)
            : $this->walkSimpleArithmeticExpression($nullIfExpression->secondExpression);

        return 'NULLIF(' . $firstExpression . ', ' . $secondExpression . ')';
    }