title: Replace and change column values while merging
body: <p>I am merging two tables created as data frame</p>
  
  <pre><code>chairs &lt;- data.frame(sales.rep = c("bob", "tom"), sales = c(5,7) )
  
  tables &lt;- data.frame(sales.rep = c("bob", "john"),sales = c(55,77) )
  
  merge(x = chairs, y = tables, by = "sales.rep", all = F)
  </code></pre>
  
  <p>the result is</p>
  
  <pre><code> sales.rep  sales.x sales.y
  1       bob       5      55
  </code></pre>
  
  <p>I have two questions.  How can I create only one column where the <code>sales.x</code> is over-written by <code>sales.y</code>?  Like following:</p>
  
  <pre><code>   sales.rep   sales
  1       bob     55
  </code></pre>
  
  <p>second question is if there is a way to do an outer join and have only one sales column with sales values coming from <code>sales.y</code> unless <code>sales.y</code> is NA.  Like the following:</p>
  
  <pre><code>  sales.rep  sales sales.y
  1       bob     55
  2       tom      7
  3      john     77
  </code></pre>
  
  <p>in other words, I only need values from <code>sales.y</code> unless it is NA.</p>
  
  <p>Any help is much appreciate in advance!</p>