title: get previous field value in trigger mysql
body: <p>I have a trigger like this in MySQL:</p>
  
  <pre><code>DELIMITER $$
  CREATE TRIGGER service_before_update BEFORE UPDATE ON service
  FOR EACH ROW
  BEGIN
      IF NEW.seatCount &lt; (?) THEN
          SIGNAL SQLSTATE '12345';
      END IF;
  END$$
  DELIMITER ;
  </code></pre>
  
  <p>instead of (?) I want previous value of "seatCount" is there any easy way or I should use something like this(ServiceNo  is a PRI key):</p>
  
  <pre><code>(SELECT seatCount FROM service WHERE serviceNo = NEW.serviceNo LIMIT 1) 
  </code></pre>