title: how to count distinct entries within a column of a data.frame
body: <p>I am good with SQL and I naturaly use sqldf package.
  However, it is useful to know native R way to achieve various SQL commands.</p>
  
  <p>on a dataframe column, how can I achieve a similar count as in the last command?</p>
  
  <pre><code>library(ggplot2)
  head(tips,3)
  sqldf("select count(distinct day) from tips")
  </code></pre>