поле 1,
поле 2,
...
ext_processed

CREATE 
	DEFINER = 'MyUser'@'%'
TRIGGER MyBD.trigger1
	BEFORE UPDATE
	ON MyBDx.receipt
	FOR EACH ROW
BEGIN
   if old.ext_processed=0 and new.ext_processed=1 then
      update receipt ss set ss.ext_date_upd_ext_processed=now();
   end if;
END

2 Can't update table 'receipt' in stored function/trigger because it is already used by statement which invoked this stored function/trigger. SQL.sql 3 8 

BEGIN
   if old.ext_processed=0 and new.ext_processed=1 then
      ##update receipt ss set ss.ext_date_upd_ext_processed=now();
      SET NEW.ext_date_upd_ext_processed = now();
   end if;
END