title: Calculate difference between values with MySQL (PV)
body: <p>I'm a bit stuck at my Bachelor's thesis and hope you can help me.
  In order to evaluate a photovoltaic system I need to calculate the difference between total energy amounts. These are automatically updated in a MySQL-table with a timestamp, but without an id-number.
  I need to get the delta/difference between those energy amounts automatically as a extra column to visualize it in Grafana.</p>
  <pre><code>******************************************
  Timestamp            | SB1_AC_Total       | Needed information (delta)
  ******************************************
  2020-06-24 09:32:45  | 11.326.302         |   23
  
  2020-06-24 09:32:02  | 11.326.279         |   22
  
  2020-06-24 09:31:20  | 11.326.257         |   ...
  </code></pre>
  <p>This list goes on for weeks.
  I really hope you can help me, because I have no idea and it is the first time I work with MySQL.</p>