title: calculate a sum of type time using sql
body: <p>How to caculate sum of times of my colonne called "timeSpent" having this format: HH:mm 
  in SQL? I am using MySQL.</p>
  
  <p>the type of my column is Time.</p>
  
  <p>it has this structure </p>
  
  <pre>
  TimeFrom like  10:00:00           12:00:00     02:00:00
  TimeUntil      08:00:00           09:15:00     01:15:00
  Time spent      
  
  total time 03:15:00
  </pre>