title: Left join using data.table
body: <p>Suppose I have two <em>data.table</em>'s:</p>
  
  <p>A:</p>
  
  <pre><code>  A  B
  1: 1 12
  2: 2 13
  3: 3 14
  4: 4 15
  </code></pre>
  
  <p>B:</p>
  
  <pre><code>   A  B
  1: 2 13
  2: 3 14
  </code></pre>
  
  <p>and I have the following code: </p>
  
  <pre><code>merge_test = merge(dataA, dataB, by="A", all.data=TRUE)
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>   A B.x B.y
  1: 2  13  13
  2: 3  14  14
  </code></pre>
  
  <p>However, I want all the rows in dataA in the final merged table. Is there a way to do this?</p>