title: NULL value count in group by
body: <p>for simplification purposes, I will use simple table attribute (meaning the table is bigger) to demonstrate the issue:</p>
  
  <p>I have the following table test:</p>
  
  <pre><code> id | nbr
  ----+-----
    1 |   0
    2 |
    3 |
    4 |   1
    5 |   1
   (5 rows)
  </code></pre>
  
  <p>id and nbr are both numeric values</p>
  
  <p>The following query </p>
  
  <pre><code>select nbr, count(nbr) from test group by nbr;
  </code></pre>
  
  <p>outputs:</p>
  
  <pre><code> nbr | count
  -----+-------
       |     0
     1 |     2
     0 |     1
  (3 rows)
  </code></pre>
  
  <p>whereas the query:  </p>
  
  <pre><code>select nbr, count(*) from test group by nbr;
  </code></pre>
  
  <p>outputs:</p>
  
  <pre><code> nbr | count
  -----+------
       |     2
     1 |     2
     0 |     1
   (3 rows)
  </code></pre>
  
  <p>I find it hard to explain the difference between count(nbr) and count(*) regarding null values
  can someone explain this to me like I'm five, thanks</p>