title: get items from one-level tree postgresql
body: <p>I have one-level tree structure like</p>
  <pre><code>create table comment (id integer not null, parent_id integer);
  </code></pre>
  <p>parent_id references to &quot;id&quot; the same table.
  I am sure that hierarchy will have not more than 1 level.
  I need to get filtered elements with their parents as list (id, parent_id)
  I tried this query, but it didn't work:</p>
  <pre><code>select *
  from comment c1
  union select * from comment c2 where c2.Id = c1.parent_id
  </code></pre>
  <p>recursive doesn't suit bcz its one level</p>