title: Calculate delta between chronological records (LibreOffice Base)
body: <p>I'm fairly new to SQL and this problem I am facing I cannot solve with my current knowledge(it's pretty impossible to become SQL expert in just few days).</p>
  
  <p>Consider that I have a following simple Table:</p>
  
  <pre><code>ID  Date        Counter
  1   03/25/14    291.5
  2   03/25/14    310
  3   03/25/14    270.9
  4   03/25/14    320
  1   04/25/14    293
  2   04/25/14    311
  3   04/25/14    278.3
  4   04/25/14    322.1
  </code></pre>
  
  <p>Basically once a month a reading of some counter is added for each user. I want to be able to create a query that would calculate delta of Counter value compared to previous month value. The result should hopefully look something like this:</p>
  
  <pre><code>ID  Date        Counter  PrevMonthDelta
  1   04/25/14    293      1.5
  2   04/25/14    311      1
  3   04/25/14    278.3    7.4
  4   04/25/14    322.1    2.1
  </code></pre>
  
  <p>I am currently experimenting in LibreOffice Base with default HSQLDB format database, but I assume this should work similar across other SQL DBs.</p>
  
  <p>Thanks in advance!</p>