title: R data.table LEFT JOIN not working properly
body: <p>So I am trying to do a LEFT join on two <code>data.table</code>s in R. Perhaps my understanding is wrong, but I thought that in a left join was supposed to fill non-matching rows with nulls or something, but in this case, I actually lose rows from my LEFT JOIN. Why?</p>
  
  <pre><code>&gt; nrow(aaa)
  [1] 30070
  &gt; ccc &lt;- merge(aaa, bbb, all.X = TRUE)
  &gt; nrow(ccc)
  [1] 23358
  </code></pre>