title: make trigger to avoid conflict
body: <p>I have booking table </p>
  
  <pre><code>id     room     date_IN         date_OUT
   1      101      29-12-2016      1-1-2017
   2      102      29-12-2016      2-1-2017
   3      101      1-1-2017        4-1-2017
  </code></pre>
  
  <p>how can I make trigger to avoid conflict date if I modify date_OUT of (id=1) to 3-1-2017  </p>