users
  user_id
  first_name
  last_name
  user_age   -- !!!! really should be stored as birthdate or birthyear

parent_user_rlt
  parent_id
  child_id


  SELECT
    grand.user_id as grandparent_id,
    parent.user_id as parent_id,
    child.user_id as child_id,
    child.first_name as child_first,
    child_last_name as child_last
  FROM users as grand
    LEFT JOIN parent_user_rlt as g2p ON g2p.parent_id = grand.user_id
    LEFT JOIN users as parent ON parent.user_id = g2p.child_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
  ;

##</code><code>##

  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 grandchildren GROUP BY grandparent_id, parent_id;
  SELECT child_id, child_first, child_last FROM grandchildren;