title: What does a double not exists clause mean?
body: <pre><code>SELECT c.name
  FROM   Customer c
  WHERE  NOT EXISTS(SELECT w.WID
                    FROM   Woker w
                    WHERE  NOT EXISTS(SELECT la
                                      FROM   look_after la
                                      WHERE  la.CID = c.CID
                                             AND la.WID = w.WID)); 
  </code></pre>
  
  <p>I dont know what the code means... Could anyone tell me broadly what the code do? C is a Customer, who will looked after from a Worker. </p>