title: SQL select COUNT issue
body: <p>I have a table</p>
  
  <pre><code>num
  ----
  NULL
  NULL
  NULL
  NULL
  55
  NULL
  NULL
  NULL
  99
  </code></pre>
  
  <p>when I wrote</p>
  
  <pre><code>select COUNT(*) 
  from tbl
  where num is null
  </code></pre>
  
  <p>the output was 7</p>
  
  <p>but when I wrote</p>
  
  <pre><code>select COUNT(num) 
  from tbl
  where num is null
  </code></pre>
  
  <p>the output was 0</p>
  
  <p>what's the difference between these two queries ?? </p>