mysql> delimiter //
mysql> create trigger upd_check before update on account 
    -> for each row
    -> begin
    -> if new.amount < 0 then
    ->  set new.amount=0;
    -> elseif new.amount > 100 then
    ->  set new.amount = 100;
    -> end if;
    -> end;
    -> //
Query OK, 0 rows affected (0.00 sec)

mysql> delimiter ;