title: negating "unknown" value in sql
body: <p>I'm currently learning SQL Server by studying the book "T-SQL Fundamentals Third Edition". In the book, there is the following quote:</p>
  
  <blockquote>
    <p>One tricky aspect of the logical value “unknown” is that when you
    negate it, you still get “unknown”. For example, given the predicate
    “not (salary > 0)”, when salary is “null”, “salary > 0” evaluates to
    “unknown”, and “not known” remain “unknown”</p>
  </blockquote>
  
  <p>I'm a bit confused by all the logic behind the statement, just wondering if anyone would clarify things a little bit? </p>
  
  <p>Would someone also add a code example?</p>