title: SQL Query: Calculating the deltas in a time series
body: <p>For a development aid project I am helping a small town in Nicaragua improving their water-network-administration.</p>
  
  <p>There are about 150 households and every month a person checks the meter and charges the houshold according to the consumed water (reading from this month minus reading from last month). Today all is done on paper and I would like to digitalize the administration to avoid calculation-errors.</p>
  
  <p>I have an MS Access Table in mind - e.g.:</p>
  
  <pre><code>*HousholdID*  *Date*     *Meter*
  0             1/1/2013   100
  1             1/1/2013   130
  0             1/2/2013   120
  1             1/2/2013   140
  ...
  </code></pre>
  
  <p>From this data I would like to create a query that calculates the consumed water (the meter-difference of one household between two months)</p>
  
  <pre><code>*HouseholdID*  *Date*     *Consumption*
  0              1/2/2013   20
  1              1/2/2013   10
  ...
  </code></pre>
  
  <p>Please, how would I approach this problem?</p>