How to create a trigger to UPDATE

As @mwasif noted, the manual covers triggers, but here is a concrete, ready approach for @welbyobeng that should work immediately.

Use a BEFORE INSERT trigger to build fullthumb while the row is being created (BEFORE triggers can change NEW values; AFTER cannot). Also add a BEFORE UPDATE trigger so changes to thumb keep fullthumb in sync.

DELIMITER //

CREATE TRIGGER files_before_insert
BEFORE INSERT ON files
FOR EACH ROW
BEGIN
  SET NEW.fullthumb = CONCAT('http://example.com/thumbs/', IFNULL(NEW.thumb, ''));
END;
//

DELIMITER ;

DELIMITER //

CREATE TRIGGER files_before_update
BEFORE UPDATE ON files
FOR EACH ROW
BEGIN
  IF NOT (NEW.thumb <=> OLD.thumb) THEN
    SET NEW.fullthumb = CONCAT('http://example.com/thumbs/', IFNULL(NEW.thumb, ''));
  END IF;
END;
//

DELIMITER ;

Notes and tips: prefer storing just the URL (not HTML) and render an anchor tag in your application layer — it keeps presentation separate from data. If some thumb values might already be full URLs, add a check (e.g., IF NEW.thumb LIKE 'http://%' OR NEW.thumb LIKE 'https://%' THEN ...) to avoid double-prefixing. If operating on a modern MySQL (5.7.6+), consider a generated column instead of triggers:

ALTER TABLE files
ADD COLUMN fullthumb VARCHAR(255) AS (CONCAT('http://example.com/thumbs/', thumb)) STORED;

Troubleshooting: if CREATE TRIGGER fails, check that the fullthumb column exists and is wide enough, that the server version supports triggers, that the client uses a proper DELIMITER, and that you have TRIGGER privileges. Test with single-row inserts first to confirm behavior.