Best merge or join function in r : <p>I have two dataframes df1 and df2.  Both have a common identifier column.</p>

<p>df1 has unique lines for each identifier. But has identifier values that are not in df2.</p>

<p>df2 has multiple lines for each identifier value.</p>

<p>I want to merge the two so that I preserve the number of rows of df2, but map the (repeating) relevant ID rows from df1 into df2.</p>

<p>Is is best to use merge or join or something else?  What arguments?</p>

<p>Thanks :)</p>
