title: Adding subqueries when one is null
body: <pre><code>SELECT (select sum(furniture.furniture_sales)
              from furniture
              where cp.userid = furniture.userid) + 
          (select sum(electronics.electronic_sales)
              from electronics
              where cp.userid = electronics.userid) as total  
  FROM customer_profile cp
  </code></pre>
  
  <p>Basically, if the userid doesn't exist in either furniture or electronics, then 'hold' will return as NULL. What can i do, so that 'total' will return a value instead of NULL?</p>