对于mysql表中的所有行，Rails计算两个日期之间的小时数

Question

假设我有这张表：

table {
    font-family: arial, sans-serif;
    border-collapse: collapse;
    width: 100%;
}

td, th {
    border: 1px solid #dddddd;
    text-align: left;
    padding: 8px;
}

tr:nth-child(even) {
    background-color: #dddddd;
}

<table>
  <tr>
    <th>id</th>
    <th>first_date</th>
    <th>second_date</th>
  </tr>
  <tr>
    <td>1</td>
    <td>2013-12-09 13:00:00</td>
    <td>2013-12-09 14:00:00</td>
  </tr>
  <tr>
    <td>2</td>
    <td>2013-12-09 13:00:00</td>
    <td>2013-12-10 13:00:00</td>
  </tr>
  <tr>
    <td>3</td>
    <td>2013-12-10 15:00:00</td>
    <td>2013-12-10 16:00:00</td>
  </tr>
</table>

对于所有行，mysql查询能否以小时为单位返回first_date和second_date之间的差值总和？我的例子是26小时？ second_date将始终大于first_date。

更新： 我现在有三种方法，必须选择：

#1. My own approach of looping through each record:

  records = Records.all
  records.each do |r|
    sum += (r.second_date-r.first_date)/1.hour
  end

#2. Using inject(thanks to slowjack2k)

   records = Records.all
   #date_diff is a model method whick substracts the two dates and returns the result
   sum = records.inject(1) {|sum, r| sum + r.date_diff }

#3. the SQL method(thanks to Shadow and slowjack2k)

   sum = Records.sum('time_to_sec(timediff(second_date, first_date))')/1.hour

性能方面，他们似乎并没有多大差异。我想我会使用SQL方法。