title: Can a WHERE clause predicate evaluate to NULL?
body: <p>Can a WHERE clause return NULL instead of TRUE or FALSE?
  According to the exercise below it is possible, but i can't imagine an example that returns NULL, Is it really possible?</p>
  
  <pre><code>4. Which of the following values can NOT be returned after evaluation of WHERE clause
  condition?
  A.  UNKNOWN
  B.  TRUE
  C.  FALSE
  D.  NULL
  Answer: A. If the result of the condition in WHERE clause is not known, NULL is returned. In all
  other scenarios, either TRUE or FALSE is returned.
  </code></pre>