title: postgres query to return some of column value
body: <p>I've the following query which return the run time for 1200 videos , i want to get the total of run time in  <code>hh:mm:ss:ms</code></p>
  
  <pre><code>select  runtime from video where ready_date between '2017-07-01' AND '2017-07-30';
  </code></pre>
  
  <p>my output will look like</p>
  
  <pre><code>"00:00:33:07"
  
  "00:00:37:09"
  
  "00:01:52:02"
  
  "00:00:41:05"
  </code></pre>
  
  <p>i've tried  sum(runtime) but looks i'm doing the wrong thing here , </p>
  
  <p>any tips ?</p>