title: how to add 2 varchar columns in sql
body: <p>I have timediff field as <code>varchar</code></p>
  
  <p>I want to find sum of this field but it gives error as </p>
  
  <p><code>"Syntax error converting the varchar value '02:00' to a column of data type int."</code></p>
  
  <p>My timediff is like</p>
  
  <pre><code>02:00
  03:00
  04:00
  </code></pre>
  
  <p>i want to add and should display <code>9</code> in sql</p>
  
  <p>please help me</p>
  
  <p>thanks for helping me</p>