MySQL遍历所有父节点和子节点

DROP TABLE IF EXISTS `t_org`; 
CREATE TABLE `t_org` (
  `id` int(11) NOT NULL,
  `name` varchar(20) DEFAULT NULL,
  `parent_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
);

INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (1, '首级组织', NULL);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (2, '一级组织', 1);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (3, '二级组织', 1);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (4, '三级组织', 1);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (5, '1-1组织', 2);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (6, '1-2组织', 2);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (7, '2-1组织', 3);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (8, '3-1组织', 4);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (9, '3-2组织', 4);
INSERT INTO t_org(`id`, `name`, `parent_id`) VALUES (10, '3-3组织', 4);

select id from (
              select t1.id,
              if(find_in_set(父级id字段名, @pids) > 0, @pids := concat(@pids, ',', 主键id字段名), 0) as ischild
              from (
                   select 主键id字段名,父级id字段名 from 表名 t  order by 父级id字段名, 主键id字段名
                  ) t1,
                  (select @pids := 需要查询的主键id) t2
             ) t3 where ischild != 0

select id from (
              select t1.id,
              if(find_in_set(parent_id, @pids) > 0, @pids := concat(@pids, ',', id), 0) as ischild
              from (
                   select id,parent_id from t_org t  order by parent_id, id
                  ) t1,
                  (select @pids := 2) t2
             ) t3 where ischild != 0

select id from (
              select t1.id,
              if(find_in_set(parent_id, @pids) > 0, @pids := concat(@pids, ',', id), 0) as ischild
              from (
                   select id,parent_id from t_org t  order by parent_id, id
                  ) t1,
                  (select @pids := 4) t2
             ) t3 where ischild != 0