title: Conditional Mapping in Talend
body: <p>I have created a simple job in Talend that will perform an inner join in the data between 2 excel sheets and then dump the result in an output excel sheet. This can be best illustrated by the below diagram :-</p>
  
  <p><img src="https://i.stack.imgur.com/DvpTv.jpg" alt="enter image description here"></p>
  
  <p>The mapping used in tMap is :-</p>
  
  <p><img src="https://i.stack.imgur.com/tAWst.jpg" alt="enter image description here"></p>
  
  <p>However the additional challenge  for me now is that I have to perform this mapping only if the column value in that row is not NULL. eg there is a mapping row1.RECID = row2.RECID, but this should only be legal if row2.RECID is not NULL.
  How do I achieve this in Talend? I have experimented a lot with tMap expressions but can't get it right..
  <br/>Here is a small sample input and it's corresponding expected output.
  Suppose my input has values :-
  v1, v2,v3,v4<br/>
  1 , A, O, 3 <br/>
  2, B, X, 4 <br/>
  3, C, X, 4 <br/></p>
  
  <p>and lookup has values <br/></p>
  
  <p>v1, v2, v3 <br/>
  1, A, O, 3<br/>
  2, null, X, 4<br/>
  3, null, C, 4<br/>
  2,null,X,null<br/></p>
  
  <p>Then the output should be :- <br/>
  v1,v2,v3<br/>
  1,A,O,3<br/>
  2,B,X,4<br/>
  2,B,X,4<br/></p>