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 | p2s3 | p2s4 | p2s4 | p4s4 | p5
 
(12 rows)

For such tables our desired query should return:
s
----s1

Thanks in advance.

-- 
Vladimir Zolotych                         gsmith@eurocom.od.ua