title: Left join only selected columns in R with the merge() function
body: <p>I am trying to LEFT Join 2 data frames but I do not want join all the variables from the second data set:</p>
  
  <p>As an example, I have dataset 1 (DF1):</p>
  
  <pre><code>  Cl    Q   Sales  Date
     A    2   30     01/01/2014
     A    3   24     02/01/2014
     A    1   10     03/01/2014
     B    4   10     01/01/2014
     B    1   20     02/01/2014
     B    3   30     03/01/2014
  </code></pre>
  
  <p>And I would like to left join dataset 2 (DF2):</p>
  
  <pre><code>Client  LO  CON
     A    12  CA
     B    11  US
     C    12  UK
     D    10  CA
     E    15  AUS
     F    91  DD
  </code></pre>
  
  <p>I am able to left join with the following code:</p>
  
  <p>merge(x = DF1, y = DF2, by = "Client", all.x=TRUE) :</p>
  
  <pre><code>   Client Q    Sales   Date             LO      CON
     A      2    30      01/01/2014       12      CA
     A      3    24      02/01/2014       12      CA
     A      1    10      03/01/2014       12      CA
     B      4    10      01/01/2014       11      US
     B      1    20      02/01/2014       11      US
     B      3    30      03/01/2014       11      US
  </code></pre>
  
  <p>However, it merges both column LO and CON. I would only like to merge the column LO.</p>
  
  <pre><code>   Client Q    Sales   Date             LO      
     A      2    30      01/01/2014       12      
     A      3    24      02/01/2014       12      
     A      1    10      03/01/2014       12      
     B      4    10      01/01/2014       11      
     B      1    20      02/01/2014       11     
     B      3    30      03/01/2014       11      
  </code></pre>