MySQL deducing fields dynamically I'm trying to deduce a value in a select statement, then reuse that calculation a few times per row, as well as group by that value. In my case, I'm doing time_to_sec(timediff(created, lastHit)) a bunch of times:

select
     time_to_sec(timediff(created, lastHit)) diff
    , if(
         time_to_sec(timediff(created, lastHit)) &gt; 1800
         OR time_to_sec(timediff(created, lastHit)) = 0,
         1, 0
      ) newSession
    , sum(1) as sessions
    , sum(
          if(
             time_to_sec(timediff(created, lastHit)) &gt; 1800, 
             0, time_to_sec(timediff(created, lastHit))
          )
      ) as totalDuration
    from pageviews
    group by newSession;


How can I improve that query without storing redundant data in my schema (such as the pre-calculated time difference between created and lastHit)? 

I thought about using variables (select @diff := timediff(...)), but this post mentions that I should avoid assigning a variable and reading from it within the same statement.
In most situations, the recalculation of an expression is much less of a performance burden than any solution.  So, don't worry about it.

You have another issue -- The GROUP BY does not include the first thing in the SELECT, so the value you will get for that is unpredictable.