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users
  user_id
...
    LEFT JOIN parent_user_rlt as p2c ON p2c.parent_id = parent.user_id
    LEFT JOIN users as child ON child.user_id = g2p.child_id
  ;

  SELECT distinct parent_id FROM grandchildren;
  SELECT grandparent_id, count(distinct parent_id) FROM grandchildren 
+GROUP BY grandparent_id;
  SELECT grandparent_id, parent_id, count(*) as num_kids FROM grandchi
+ldren GROUP BY grandparent_id, parent_id;
  SELECT child_id, child_first, child_last FROM grandchildren;