求下级所有节点
SELECT b.* FROM tree AS a, tree AS b WHERE a.id= b.pid AND(a.id= ? OR a.pi= ?)
求所有子节点
SELECT * FROM tree t WHERE NOT EXISTS( SELECT 1 FROM tree t1, tree t2 WHERE t1.id= t2.pid AND t1.id= t.id )
求所有根节点
SELECT * FROM tree t WHERE NOT EXISTS( SELECT 1 FROM tree t1, tree t2 WHERE t1.id= t2.pid AND t1.id= t.pid )