title: Keeping rows from double-counting in a GROUP BY
body: <p>Here's the basic guts of my schema and problem: <a href="http://sqlfiddle.com/#!1/72ec9/4/2" rel="nofollow noreferrer">http://sqlfiddle.com/#!1/72ec9/4/2</a></p>
  <p>Note that the periods table can refer to a variable range of time - it could be an entire season, it could be a few games or one game.  For a given team and year all period rows represent exclusive ranges of time.</p>
  <p>I've got a query written which joins up tables and uses a GROUP BY periods.year to aggregate scores for a season (see sqlfiddle).  However, if a coach had two positions in the same year the GROUP BY will count the same period row twice.  How can I ditch the duplicates when a coach held two positions but still sum up periods when a year is comprised of multiple periods?  If there's a better way to do the schema I'd also appreciate it if you pointed it out to me.</p>