title: nested select queries - mysql sakila database
body: <p>I have found the minimum postode for each district, and now want to find the maximum of all of these district postcode minimums. Im getting stuck with how to do max while keeping the other parts in tact.</p>
  
  <pre><code>select 
     district, 
     postal_code, 
     min(postal_code) from address
  group by district
  having district is not null and district !="" and postal_code&gt;0
  order by district;
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/wwh2f.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wwh2f.png" alt="result"></a></p>