title: merge command comparison between R and Stata
body: <p>Being a R user, I am learning Stata now using <a href="http://www.cpc.unc.edu/research/tools/data_analysis/statatutorial/example11" rel="nofollow">this</a> resource, and am puzzled about the <code>merge</code> command.</p>
  
  <p>In R, I don't have to worry about merging data wrongly, because it merges everything anyway. I don't need to worry if the common columns contain any duplicates, because the <code>Y</code> dataframe will merge to each of the duplicated row in <code>X</code> dataframe. (using <code>all=FALSE</code> in <code>merge</code>)</p>
  
  <p>But for Stata, I need to remove the duplicate rows from <code>X</code> before proceeding to merge.</p>
  
  <p>Is it being assumed in Stata that, in order for <code>merge</code> to proceed, the common column in the master table must be unique?</p>