title: Get difference since 30 days ago in InfluxQL/InfluxDB
body: <p>I have a single stat in my grafana dashboard showing the current usage of a disk. To get that info I use the following query:</p>
  
  <pre><code>SELECT last("used") FROM "disk" WHERE "host" = 'server.mycompany.com' 
  AND "path" = '/dev/sda1' AND $timeFilter
  </code></pre>
  
  <p>I want to add another stat showing the increase/decrease in usage over the last 30 days. I assume for this I want to get the last measurement and the measurement from 30 days ago and subtract them. </p>
  
  <p>How can I do this in InfluxQL?</p>