title: Using ISNULL vs using COALESCE for checking a specific condition?
body: <p>I know that multiple parameters can be passed to <code>COALESCE</code>, but  when you want to to 
  check just one expression to see if it doesn't exist, do you use a default or is it a better practice to use <code>ISNULL</code> instead? </p>
  
  <p>Is there any performance gain between the two?</p>