title: R: Merging two dataframes without multiplying rows
body: <p>I have two dataframes which I would like to merge:</p>
  
  <p>df1:</p>
  
  <pre><code>Day   name 
  1     A   
  1     B     
  1     C     
  2     A   
  2     B      
  2     C    
  3     A      
  3     B   
  </code></pre>
  
  <p>df2:</p>
  
  <pre><code>Day   spending
  2     25
  2     30
  3     20
  3     15
  3     10
  </code></pre>
  
  <p>Desired outcome:</p>
  
  <pre><code>Day   name   spending
  1     A      NA
  1     B      NA
  1     C      NA
  2     A      25   
  2     B      30
  2     C      NA
  3     A      20
  3     B      15
  3     NA     10 
  </code></pre>
  
  <p>I have tried merge &amp; join_all, but all I got was:</p>
  
  <pre><code>Day   name   spending
  1     A      NA
  1     B      NA
  1     C      NA
  2     A      25 
  2     A      30
  2     B      25      
  2     B      30
  2     C      NA
  3     A      20
  3     A      15
  3     A      10
  3     B      20
  3     B      15
  3     B      10
  3     NA     20
  3     NA     15
  3     NA     10
  </code></pre>
  
  <p>Note that I wish to join the two dataframes by day without having the length of rows multiplying at all, despite missing data in some columns (such as day 1 data in df2 &amp; missing name of C in df1). Please advise</p>