title: Calculations of different columns in Mysql Query
body: <p>I have a Table:-</p>
  
  <pre><code>+-----+--------------+--------------+----------+--------------------+---------------+-----------------+
  | id  | CustomerName | VideoQuality | IsActive | BufferedTime       | ElapsedTime   | TotalBufferTime |
  +-----+--------------+--------------+----------+--------------------+---------------+-----------------+
  | 139 | HotStar      | 180          | Yes      | 10.367167126617211 | 30.000000000  | NULL            |
  | 140 | HotStar      | 1300         | NULL     | 5.43524230876729   | 34.000000000  | NULL            |
  | 141 | HotStar      | 1300         | NULL     | 5.671054515212042  | 38.000000000  | NULL            |
  | 142 | HotStar      | 1300         | NULL     | 5.045639532902047  | 41.000000000  | NULL            |
  | 143 | HotStar      | 1300         | NULL     | 5.455747718023355  | 44.000000000  | NULL            |
  | 144 | HotStar      | 1300         | NULL     | 5.691559924468107  | 49.000000000  | NULL            |
  </code></pre>
  
  <p>i want to calculate the columns BufferTime and ElapsedTime and insert that output to TotalBufferTime column but i want to skip the first row of the BufferTime.
  So the fisrt calculation will be 5.43 + 30.000 second calculation will be 5.67 + 34.00 and so on.
  I also have a column IsActive which shows the first row of Buffer time.</p>
  
  <p>I want to do something like this :-</p>
  
  <pre><code>update RequestInfo SET `TotalBufferTime` = BufferedTime + ElapsedTime;
  </code></pre>
  
  <p>only thing i want to skip only the first row of the column buffered time.</p>