title: MySQL - SUM of a group of time differences
body: <p>I want to sum all the time differences to show the total hours worked by a volunteer. Getting a result set of time differences is easy:</p>
  
  <pre><code>Select timediff(timeOut, timeIn) 
  FROM volHours 
  WHERE username = 'skolcz'
  </code></pre>
  
  <p>which gives the list of times by hours but then I want to sum it up to a grand total.</p>
  
  <p>So if the result set is:</p>
  
  <pre><code>12:00:00
  10:00:00
  10:00:00
  08:00:00
  </code></pre>
  
  <p>It would just total 40 hours.</p>
  
  <p>This there a way to do something like:</p>
  
  <pre><code>SELECT SUM(Select timediff(timeOut,timeIn) 
  FROM volHours 
  WHERE username = 'skolcz') as totalHours
  </code></pre>
  
  <p>?</p>