title: SQL - difference and average between more records
body: <p>I have the following table COUNTRY_PEOPLE:</p>
  
  <pre><code>COUNTRY - CITY - YEAR - PEOPLE
  
  ENGLAND - LONDON 1980 - 7834020
  ENGLAND - LONDON 2010 - 8308369
  ENGLAND - DERBY  1980 - 231483
  ENGLAND - DERBY  2010 - 233700
  FRANCE  - PARIS  1980 - 2174654
  FRANCE  - PARIS  2010 - 2274880
  FRANCE  - NANTES 1980 - 279321
  FRANCE  - NANTES 2010 - 290130
  </code></pre>
  
  <p>I need a sql query to get for each country the list of city with the difference between the people in the 2010 and 1980.
  So:</p>
  
  <pre><code>ENGLAND - LONDON  474349
  ENGLAND - DERBY   2217
  FRANCE  - PARIS   100226
  FRANCE  - NANTES  10809
  </code></pre>
  
  <p>Then the average for country, but this should be easy with a group by.
  I just invented the example as translated from a real boring scenario, all data are fake.
  Thank you in advance </p>