delimiter $

drop table if exists a1; $
create table a1(value text COLLATE utf8_unicode_ci); $
insert into a1 values('12345'); $

drop trigger if exists a1_before_update; $
create trigger a1_before_update before update on a1 for each row
begin
    declare _value text;
    set _value = '1234567';
    set new.value = _value; 
end;$

update a1 set value = '';$
select value from a1;