is this a bug in PG 7?

In PG 7.0.2, I have a select:

SELECT pr_category_id, pr_category_id FROM pr_category
    UNION SELECT pr_category_id, parent_cat FROM pr_category WHERE parent_cat
<> 0
    UNION SELECT pr_category_id, parent_cat_link FROM pr_category_link

when I run this I get a subset of the correct results. If I move the first
select to the end as in:

SELECT pr_category_id, parent_cat FROM pr_category WHERE parent_cat <> 0
    UNION SELECT pr_category_id, parent_cat_link FROM pr_category_link
    UNION SELECT pr_category_id, pr_category_id FROM pr_category

then it works correctly.

Any ideas? Is this a bug?


Thanks,


--

\\\\\    ///    Rainer Mager
 \\\\\  ///
  \\\VANGUARD   Ever Green Bldg., 5F
   \\\///       3-3-2 Naka-Meguro, Meguro-ku, Tokyo 153-0061
    \\//        Phone: 5768-2855  Fax: 5768-2856
     \/         Email: rmager@vgkk.co.jp  Web: www.vgkk.co.jp