title: Count unique values in Stata
body: <p><code>codebook</code> is a great command in Stata. It describes data contents but also simply identifies unique values</p>
  
  <pre><code>sysuse auto, clear
  codebook mpg, compact
  </code></pre>
  
  <p>Number of unique values of mpg is 21. Looking at the help of the command, it does not seem possible to store this value. Am I wrong?</p>
  
  <p>I am aware of other ways to compute the <a href="http://www.stata.com/support/faqs/data-management/number-of-distinct-observations/" rel="noreferrer">number of unique values</a> in Stata, but it would be so convenient to add this feature to the <code>codebook</code> command.</p>