title: Calculating percent change from one record to next using a SQL query
body: <p>I am using this query to calculate percent change of Enrollment total from current semester to the last. What changes do I need to make to this query to avoid nulls ?
  I have tried using a sub query which is painfully slow.</p>
  
  <pre><code>SELECT E2.Term AS Prev_Term, 
         E2.[Enrollment Total] AS Prev_Enrl_Tot, 
         E1.Term,
         E1.Location, 
         E1.milestone_description, 
         E1.polling_date,
         Round(((E1.[Enrollment Total] - [Prev_Enrl_Tot]) 
                                     / [Prev_Enrl_Tot]) * 100,2) AS Percent_Change
  FROM EnrollmentsByLocation E1 
  LEFT OUTER JOIN EnrollmentsByLocation E2
  ON E1.Location = E2.Location 
  AND E1.milestone_description = E2.milestone_description 
  AND E1.Term - E2.Term = 1;
  </code></pre>