Any way to optimize? To ensure that its working on MySQL, SQLITE, Postgre SQL?

campaign-next-number.rb

  # Find next number to send call
  def next_number
    # first parse CDRs
    Cdr.parse

    # find number
    sql_request = %{campaign_id = ? and
      call_state = ? and
      number_of_retries < ? and
      (UNIX_TIMESTAMP(NOW()) - UNIX_TIMESTAMP(updated_at)) <= ?
    }.gsub(/\s+/," ").chomp.strip

    CampaignNumber.where(sql_request, self.id, "idle",
      amount_of_max_attempts,
      minimal_timeout_between_retrials
    ).order("number_of_retries, updated_at").first
  end