Need help in composing PostgreSQL query

Hi

Please help me compose the query in PostgreSQL.
Using PostgreSQL 7.1.2.

Suppose relations A and B have columns:
{X1, X2, ..., Xm, Y1, Y2, ..., Yn}
and
{Y1, Y2, ..., Yn}
Attributes Y1, Y2, ..., Yn are common for both relations
and have the same type in both.

How can I define in PostgreSQL the query producing
relation with columns X1,X2,...,Xm containing all those tuples
satisfying conditon: relation A contains tupple
{x1,x2,...xm,y1,y2,...,yn}
for _each_ tupple
{y1,y2,...,yn}
in relation B ? Where x1 denotes particular value of
colum X1 etc.

For example: consider two tables DEND and DOR.

DEND DOR

s | p p
----+---- ----
s1 | p1 p1
s1 | p2 p2
s1 | p3 p3
s1 | p4 p4
s1 | p5 p5
s1 | p6 p5
s2 | p1 (6 rows)
s2 | p2
s3 | p2
s4 | p2
s4 | p4
s4 | p5
(12 rows)

For such tables our desired query should return:

s
----
s1

Thanks in advance.

--
Vladimir Zolotych gsmith(at)eurocom(dot)od(dot)ua