您可以按以下步骤操作：

SELECT user_id 
FROM club_tree
WHERE user_id NOT IN (
    SELECT parent_id
    from club_tree
    where club = 'club1'
    group by parent_id 
    HAVING count(1) >= 2
) 
and club = 'club1';

我们选择不在具有两个以上子女的用户列表中的用户。
这是为了让拥有两个以上孩子的用户：

SELECT parent_id
    from club_tree
    group by parent_id 
    HAVING count(1) >= 2

您可以在这里测试：http://sqlfiddle.com/#!9/eb2c70/3

您可以使用left join尝试以下查询吗：

SELECT c.user_id
  from club_tree c
  left join 
  (
    select parent_id
    from club_tree
    where club = 'club1'
    group by parent_id 
    HAVING count(1) >= 2
  ) as c2 on c.user_id = c2.parent_id
  where c.club = 'club1' and c2.parent_id is null;

在这里试试：http://sqlfiddle.com/#!9/eb2c70/17