title: Is it possible to relate two dataframes (eg: States and cities)?
body: <p>I have three dataframes: </p>
  
  <p><code>cities_df</code>, which contains the name of a city amongst other fields </p>
  
  <pre><code>cities_df &lt;- data.frame(
    city_name = c("London", "Newcastle Upon Tyne", "Gateshead"),
    city_population = c(8673713L, 289835L, 120046L),
    city_area = c(1572L, 114L, NA)
  )
  </code></pre>
  
  <p><code>states_df</code>, which contains the name of a state amongst other fields </p>
  
  <pre><code>states_df &lt;- data.frame(
    state_name = c("Greater London", "Tyne and Wear"),
    state_population = c(123, 456)
  )
  </code></pre>
  
  <p><code>dictionary_df</code>, which contains the whole list of cities and their corresponding state.</p>
  
  <pre><code>dictionary_df &lt;- data.frame(
    city_name = c("London", "Newcastle Upon Tyne", "Gateshead"),
    state = c("Greater London", "Tyne and Wear", "Tyne and Wear")
  )
  </code></pre>
  
  <p>Is there any way to relate/link <code>cities_df</code> and <code>states_df</code> dataframes so I can have an easy way to get all the cities' fields that belong to a certain state?</p>