Problem with complex join

Hi!

   I ran a query:
SELECT p.subsec_id, p.pos_id, cn.pos_id
   FROM central cn, shops sh, districts d, positions p
      WHERE cn.shop_id = sh.shop_id AND sh.distr_id = d.distr_id
      AND   d.city_id = 1 ;

and got a huge list, where, are, e.g:
subsec_id|pos_id|pos_id
---------+------+------
        1|     1|     1
        1|     1|     1
        1|     1|     1
        1|     1|     1
   [skipped]
        1|     2|     1
        1|     2|     2
        1|     2|     2
        1|     2|     2

and so on.

   I modified the query to exclude rows:

SELECT p.subsec_id, p.pos_id, cn.pos_id
   FROM central cn, shops sh, districts d, positions p
      WHERE cn.shop_id = sh.shop_id AND sh.distr_id = d.distr_id
      AND   d.city_id = 1 AND cn.pos_id = p.pos_id ;

but got 0 rows as a result. I expected:
subsec_id|pos_id|pos_id
---------+------+------
        1|     1|     1
   [skipped]
        1|     2|     2

   Is it a bug?

Oleg.
---- 
    Oleg Broytmann     http://members.xoom.com/phd2/     phd2@earthling.net
           Programmers don't die, they just GOSUB without RETURN.