title: Multiple usage of same formula in SELECT fields
body: <p>I have a query where I calculate value for WorkingTime as follows:</p>
  
  <pre><code> SELECT
      SUM(CASE WHEN a.Type = 1
                  THEN CAST(DATEDIFF(s,
                                  ca.StartTime,
                                  ca.EndTime) AS FLOAT)
                      / 60.0
                  ELSE NULL
          END) AS WorkingTime ,
      AnnotherResult ...
  </code></pre>
  
  <p>The value of AnotherResult is also calculated with some equation, and in this equation I need to use WorkingTime.</p>
  
  <p>Do I need to repeat the SUM (CASE ....) which is used to calculate WorkingTime everywhere I require it, or I can somehow refer directly to WorkingTime value in subsequent field value calculation?</p>