beginners trigger problem

hello,

i have following table:
id b_stueck b_name b_preis b_summe
(units) (price) (sum)

i want b_summe automatically be calculated, therefore i use
following trigger:
CREATE RULE summieren AS ON INSERT TO t_bestand DO UPDATE t_bestand SET
b_summe=(b_stueck*b_preis) FROM t_bestand

after all, i get following result:

webliste=# INSERT INTO t_bestand(b_stueck, b_name, b_preis)
VALUES(1,'CD','20');
INSERT 0 2
webliste=# SELECT * FROM t_bestand;
id | b_stueck | b_name | b_preis | b_summe
----+----------+------------+---------+----------
1 | 3 | Festplatte | DM49,00 | DM147,00
2 | 4 | Disketten | DM45,00 | DM180,00
5 | 1 | CD | DM20,00 |
6 | 1 | CD | DM20,00 |
(4 rows)

the row is doubled,

any hints why and how the b_summe be calculated automatically??

thanks in advance
--
Hakan Kuecuekyilmaz FHT-Esslingen University of Applied Sciences,
Germany
hakubw00(at)fht-esslingen(dot)de