title: whether merge and Reduce doing the same thing and all=True and by can reach the same goal
body: <p>I am try to merge 4 data together. I tried <code>merge</code> and <code>reduce</code>. I wonder whether I am using them in the correct way. The outcome data seems like the same. I will need second eye to make sure I am not doing it wrong.</p>
  <p>Here is my codes:</p>
  <p>I used merge:</p>
  <pre><code>SUB_counts &lt;- as_tibble(merge(count_pts, count_SUB, all = TRUE, sort = FALSE)) 
  SUB_counts_Re &lt;- as_tibble(merge(count_pts_Re, count_SUB_Re, all = TRUE, sort = FALSE)) 
  SUB_counts_merge &lt;- full_join(count_pts, count_SUB,  by = &quot;SUBCOD&quot;)
  </code></pre>
  <p>and then I also tried <code>Reduce</code>:</p>
  <pre><code>test&lt;-Reduce(function(x, y) merge(x, y, all=TRUE), list(count_pts, count_SUB, count_pts_Re, count_SUB_Re)) 
  </code></pre>
  <p>It looks like test=SUB_counts_merge</p>
  <p>Could someone help me better understand those two?</p>
  <ol>
  <li>when we use <code>all = TRUE</code>, is it same as use <code> by = &quot;SUBCOD&quot;</code>?</li>
  <li>when we use <code>Reduce</code>, there is no place to put 'by`, how can we know they are merged using the correct variable?</li>
  </ol>
  <p>many thanks.</p>