Getting one row for each subquery row...?

adding comments to a tableHi All,
here are my three tables. I wold like to list them in the following way: I need all the columns from t_stockchanges, and one field for the pgoductgroup the t_stockchanges.productid belongs to.

But one product can belong to many group, so i would need one row for each group for each product.

My bad query is:
------------------------
DB=# select t_stockchanges.productid, (select name from t_productgroups where id=(select productgroupid from t_prod_in_pgr where productid=t_stockchanges.productid)) as pgroup from t_stockchanges;
ERROR: More than one tuple returned by a subselect used as an expression.
DB=#
---------------
Yes, this is absolutelly true, but I would like postgres to give me all the tuples found. How can I ask him to do so?

Thank you,
-- Csaba

--------------------------------------------------------------------------------------------------------------------------------
Table "public.t_stockchanges"
Column | Type | Modifiers
---------------+------------------+-----------------------------------------------------
id | integer | not null
stockid | integer | not null
productid | integer | not null
changeid | integer | not null
quantity | double precision | not null
date | character(19) | not null
purchaseprice | double precision | not null
correction | double precision | not null
userid | integer | not null
time | character(19) | default to_char(now(), 'YYYY.mm.dd hh:mi:ss'::text)
prooftype | character(10) | not null default ''
proofid | integer | default 0
Indexes: t_stockchanges_pkey primary key btree (id),
t_stockchanges_date btree (date),
t_stockchanges_productid btree (productid)
--------------------------------------------------------------------------------------------------------------------------------

Table "public.t_productgroups"
Column | Type | Modifiers
-------------+-----------------------+-----------
id | integer | not null
name | character varying(30) | not null
description | character varying |
root | boolean |
Indexes: t_productgroups_pkey primary key btree (id)
--------------------------------------------------------------------------------------------------------------------------------

Table "public.t_prod_in_pgr"
Column | Type | Modifiers
----------------+---------+-----------
productgroupid | integer | not null
productid | integer | not null
--------------------------------------------------------------------------------------------------------------------------------

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