title: sql query subtraction
body: <p>I have a query where it takes my total sales and subtracts by my canceled sales, however if there are no canceled sales the value is null. then when the </p>
  
  <p>so basically if my canceledsales where null it returns a null value for the subtraction column instead of the totalsales</p>
  
  <pre><code>totalsales-canceledsales(null) = null
  1000-null=null
  </code></pre>
  
  <p>i want it to be like this</p>
  
  <pre><code>1000-null=1000
  </code></pre>