title: Create a list of unique values from a column in multiple data frames in R
body: <p>Suppose I have 3 data frames (df1, df2, df3) that explain the color, year and make of a car.  Each data frame has a column named "id".  Not all of the ids match in each data frame, but I want to create a list of unique ids across all three data frames.</p>
  
  <pre><code> df1 looks like this:  
  
   id     color
   5A     black
   7T     green
   8Q     gold
   4D     white
  
   df2 looks like this:  
  
   id     make
   5A     BMW
   6N     Benz
   8Q     GM
   3G     Toyota
  
   df3 looks like this:  
  
   id     year
   5A     2003
   2B     2010
   8Q     1999
   4D     2007
  </code></pre>
  
  <p>Here's the code to make the 3 data frames:</p>
  
  <pre><code> df1 &lt;- data.frame(id=c("5A", "7T", "8Q", "4D"), color=c("black", "green", "gold", "white"))
   df2 &lt;- data.frame(id=c("5A", "6N", "8Q", "3G"), make=c("BMW", "Benz", "GM", "Toyota"))
   df3 &lt;- data.frame(id=c("5A", "2B", "8Q", "4D"), year=c(2003,2010,1999,2007))
  </code></pre>
  
  <p>I would like my final list to have the following values:</p>
  
  <pre><code> id
   5A
   7T
   8Q
   4D
   6N
   3G
   2B
  </code></pre>
  
  <p>I tried using unique() but that only works for one vector?  Any thoughts?</p>