title: Addition between two column of two different table
body: <p>Suppose I have two tables as <code>SEM1</code> &amp; <code>SEM2</code>.</p>
  
  <p><code>Student_id</code> is common column between them. I have applied <strong>left join between two columns</strong> and I am adding students marks from both column.</p>
  
  <p>If table <code>SEM1</code>  have record for particular student  having marks as <code>20</code>, but <code>SEM2</code> doesn't have record for that particular student then it should do addition as <code>20+0</code>.<br>
  <strong>How to fire that query</strong>?</p>