title: Counting null and non-null values in a single query
body: <p>I have a table </p>
  
  <pre><code>create table us
  (
   a number
  );
  </code></pre>
  
  <p>Now I have data like:</p>
  
  <pre><code>a
  1
  2
  3
  4
  null
  null
  null
  8
  9
  </code></pre>
  
  <p>Now I need a single query to count null <strong>and</strong> not null values in column a </p>