title: cannot create triggers on objects owned by SYS issue
body: <p>I log in with a user (not sys) from sql plus as in the first picture and run the trigger from oracle sql developer but it gives me error as in the second picture.</p>
  
  <p><a href="https://i.stack.imgur.com/mvDx3.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mvDx3.png" alt="enter image description here"></a></p>
  
  <p><code>ORA-04089: cannot create triggers on objects owned by SYS</code></p>
  
  <p><a href="https://i.stack.imgur.com/vFFkR.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vFFkR.png" alt="enter image description here"></a></p>
  
  <p>I'm using Oracle 12c and not sure whether my trigger code is exactly correct.  My purpose is to show a warning when someone tries adding same ogrenci_no (student_number). What can I do for this?</p>