title: MYSQL - can a extra column is forced to update every time
body: <p>i have a table with 3 column A, B, &amp; C.
  when ever some one update any column(A or B or both) it should update column C also. </p>
  
  <p>Means someone cann't just write "update tableName set a=1, b=2 where blah blah". 
  It should be always <strong>"update tableName set a=something, b=something2, c=something3 where blah blah"</strong>.
  if someone don't do it, it returns some error.</p>
  
  <p>i tried using trigger. But it didn't help me out. Below is the trigger that executed before update.</p>
  
  <pre><code>BEGIN 
    DECLARE TIMESTAMP DATE DEFAULT now();
    DECLARE msg VARCHAR(255);
    IF NEW.C IS NULL OR NEW.C = "" THEN
      SET msg = concat("Trying TO UPDATE NULL OR empty VALUE of C.");
      SIGNAL SQLSTATE "4500" SET MESSAGE_TEXT = msg;
    END IF;
  END
  </code></pre>
  
  <p>New.C give the Existing Value of C in row</p>