-- 使用 PostgreSQL 
-- create trigger 時有 for each row
-- 這樣就能避免上面的情況
-- 提供你參考

create table ithelp190822 (
  id int generated always as identity primary key
, mg001 text not null
, mg002 text not null
, mg201 numeric(12,6) not null
);

insert into ithelp190822 (mg002, mg001, mg201)
values
('6400VV', '2423118', 333.000000),
('6400VV', '2423927', 333.000000);

insert into ithelp190822 (mg002, mg001, mg201)
values
('#6300VV', '2423927', 333.000000);

--
create or replace function trifun_ithelp190822()
returns trigger
as
$$
begin
    if left(new.mg002, 1) = '#'
    then
        raise exception '不可修改#開頭的項目';
    end if;
    return new;
end;
$$ language plpgsql;

create trigger tri_bu_ithelp190822
before update on ithelp190822
for each row execute procedure trifun_ithelp190822();

--
[miku]# \d ithelp190822
                           Table "miku.ithelp190822"
+--------+---------------+-----------+----------+------------------------------+
| Column |     Type      | Collation | Nullable |           Default            |
+--------+---------------+-----------+----------+------------------------------+
| id     | integer       |           | not null | generated always as identity |
| mg001  | text          |           | not null |                              |
| mg002  | text          |           | not null |                              |
| mg201  | numeric(12,6) |           | not null |                              |
+--------+---------------+-----------+----------+------------------------------+
Indexes:
    "ithelp190822_pkey" PRIMARY KEY, btree (id)
Triggers:
    tri_bu_ithelp190822 BEFORE UPDATE ON ithelp190822 FOR EACH ROW EXECUTE PROCEDURE trifun_ithelp190822()
    
---
update ithelp190822
set mg201 = 555
where mg002 = '6400VV';

UPDATE 2
Time: 3.967 ms
[miku]# commit;
COMMIT
Time: 2.298 ms
[miku]# select * from ithelp190822;
+----+---------+---------+------------+
| id |  mg001  |  mg002  |   mg201    |
+----+---------+---------+------------+
|  3 | 2423927 | #6300VV | 333.000000 |
|  1 | 2423118 | 6400VV  | 555.000000 |
|  2 | 2423927 | 6400VV  | 555.000000 |
+----+---------+---------+------------+
(3 rows)

update ithelp190822
set mg201 = 666
where mg002 = '#6300VV';

ERROR:  P0001: 不可修改#開頭的項目

select * from ithelp190822;
+----+---------+---------+------------+
| id |  mg001  |  mg002  |   mg201    |
+----+---------+---------+------------+
|  3 | 2423927 | #6300VV | 333.000000 |
|  1 | 2423118 | 6400VV  | 555.000000 |
|  2 | 2423927 | 6400VV  | 555.000000 |
+----+---------+---------+------------+

MS SQL 可以這樣寫，參考看看

IF EXISTS
	(
		SELECT * 
		FROM inserted
		WHERE LEFT(MG002 , 1) = '#'
	)
	BEGIN
		RAISERROR('不可修改 # 開頭的項目' , 16 , 10)
	END