title: SQL foreign key constraint and dangling tuples
body: <blockquote>
    <p>A tuple with a foreign key value that does not appear in the referenced relation is said to be a dangling tuple.</p>
  </blockquote>
  
  <p>I found this in a book. Can anyone explain this?</p>
  
  <p>If we have a <strong>table</strong></p>
  
  <pre><code>CREATE TABLE Studio (
  name CHAR(30) PRIMARY KEY,
  address VARCHAR(255),
  presC# INT,
  FOREIGN KEY (presC#) REFERENCES MovieExec(cert#)
  );
  </code></pre>
  
  <p>Then we can't add a tuple with a <code>presC#</code> that doesn't exist in <code>MovieExec</code>. So how can they get dangling tuples?</p>