title: Converting NOT IN to NOT EXISTS
body: <p>Having a nightmare of a time understanding the usage of NOT EXISTS, primarily how to convert my NOT IN solution below so that I can actually understand how I achieved the results.
  Have several articles on askTom, the oracle forums and stackoverflow, but can't find anything that clearly helps understand this problem. My apologies if I have missed it through my noobish searching.</p>
  
  <pre><code>SELECT s.S_Fname, s.S_Lname
  FROM STUDENT s
  WHERE s.S_Sex = 'F'
  AND S.S_Id NOT IN(SELECT e.S_Id
          FROM ENROLLMENT e
          WHERE e.Mark &lt; 70);
  </code></pre>
  
  <p>Bit of assistance with the content, trying to find female students who have never received a mark below 70 in any class they have been enrolled in.</p>