title: Return all data if variable is NULL
body: <p>I'm trying to write a query that returns data based on a user input variable (@VariantID).</p>
  
  <pre><code>WHERE sod.ProductVariantID = @VariantID
  </code></pre>
  
  <p>The problem is that if @VariableID is NULL, I want to return all data regardless of what sod.ProductVariantID equals. </p>
  
  <p>What's the best way of doing this?</p>