title: Joining pandas DataFrames by Column names
body: <p>I have two DataFrames with the following column names:</p>
  <pre><code>frame_1:
  event_id, date, time, county_ID
  
  frame_2:
  countyid, state
  </code></pre>
  <p>I would like to get a DataFrame with the following columns by joining (left) on <code>county_ID = countyid</code>:</p>
  <pre><code>joined_dataframe
  event_id, date, time, county, state
  </code></pre>
  <p>I cannot figure out how to do it if the columns on which I want to join are not the index.</p>