title: Trying to understand "except all" in sql query
body: <p>I came across this example and I don't understand what it means. </p>
  
  <pre><code>(SELECT drinker FROM Frequents)
       EXCEPT ALL
  (SELECT drinker FROM Likes);
  </code></pre>
  
  <p>relations: Frequents(drinker, bar), Likes(drinker, beer)</p>
  
  <p>What does the ALL do in this case? How is the result different from the query below?</p>
  
  <pre><code>(SELECT drinker FROM Frequents)
       EXCEPT
  (SELECT drinker FROM Likes);
  </code></pre>