title: Django: Calculate and Group inside Django ORM with multiple columns
body: <p>Good day,</p>
  <p>right now I'm trying to improve my knowledge about Django ORM but struggling with the task below:</p>
  <p>But first, the database looks like this:</p>
  <pre><code>class DeathsWorldwide(models.Model):
      causes_name = models.CharField(max_length=50, null=False, blank=False)
      death_numbers = models.PositiveIntegerField(default=0)
      country = models.CharField(max_length=50, null=True, blank=True)
      year = models.PositiveIntegerField(null=False, blank=False, validators=[MinValueValidator(1990), MaxValueValidator(2019)])
  
  
  causes_name    |    death_numbers    |    country    |    year
  Alcohol dis.   |    25430            |    Germany    |    1998
  Poisoning      |    4038             |    Germany    |    1998
  ...
  Maternal dis.  |    9452             |    Germany    |    1998
  Alcohol dis.   |    21980            |    Germany    |    1999
  Poisoning      |    5117             |    Germany    |    1999
  ...
  Maternal dis.  |    8339             |    Germany    |    1999
  </code></pre>
  <p>Always a block of all diseases for each year, every country and so on...The range of years goes from 1990 to 2019.</p>
  <p>What I - or rather let's say the task - want to achieve, is a list of all countries with calculated numbers of deaths, like this...</p>
  <pre><code>country    |    death_numbers
  France     |    78012
  Germany    |    70510
  Austria    |    38025
  </code></pre>
  <p>...but with one additional feature: the number of deaths for each country between 1990-1999 must be subtracted from those of 2000-2019. So a full list would actually look something like this:</p>
  <pre><code>country    |    death_numbers    |    19xx    |    2xxx
  France     |    78012            |    36913   |    114925
  Germany    |    70510            |    ...     |    ...
  Austria    |    38025            |    ...     |    ...
  </code></pre>
  <p>Is it possible to achieve such a result with only one query?</p>
  <p>Thanks for your help and have a great day!</p>