title: Using SUM so that NULL in columns make sum NULL
body: <p>I have a a table with a column groups INTEGER NULL. It has values</p>
  
  <h2>groups</h2>
  
  <p>5<br>
  7<br>
  <code>&lt;NULL&gt;</code>  </p>
  
  <p>If I do a select sum(groups) form table_name<br>
  I would get 12. How can I get null, when  the column being summed has a null.</p>