title: R: multiple merge with big data frames
body: <p>I have two big dataframes: DBa and DBb. All colums of DBb are in DBa. </p>
  
  <p>I want to merge these two dataframes by all DBb's colums. </p>
  
  <p>I'm trying:</p>
  
  <pre><code>new &lt;- merge(DBa, DBb, by=colnames(DBb)) 
  </code></pre>
  
  <p>but it gives me the error:</p>
  
  <pre><code>Elements listed in `by` must be valid column names in x and y
  </code></pre>
  
  <p>How can I do it?</p>