SQL 查询父子节点并限制行数的写法？

[Kaciras]

数据库里有一张 tree 表保存了树结构的数据，有 id 和 parent 两列，子节点的 parent 等于父节点的 id，没有父节点的 parent=0。

能否做到在一次查询中获取：

• 前两个 parent=0 的节点
• 上一步查询出来的每个节点的前两个子节点

比如:

INSERT INTO "tree" ("id", "parent") VALUES (1, 0);
INSERT INTO "tree" ("id", "parent") VALUES (2, 1);
INSERT INTO "tree" ("id", "parent") VALUES (3, 1);
INSERT INTO "tree" ("id", "parent") VALUES (4, 1);
INSERT INTO "tree" ("id", "parent") VALUES (5, 0);
INSERT INTO "tree" ("id", "parent") VALUES (6, 5);
INSERT INTO "tree" ("id", "parent") VALUES (7, 5);
INSERT INTO "tree" ("id", "parent") VALUES (8, 5);
INSERT INTO "tree" ("id", "parent") VALUES (9, 5);
INSERT INTO "tree" ("id", "parent") VALUES (10, 0);

查询出结果应该是（顺序无所谓）：

+-----+--------+
| id  | parent |
+-----+--------+
|  1  |   0    |
|  5  |   0    |
|  2  |   1    |
|  3  |   1    |
|  6  |   5    |
|  7  |   5    |
+-----+--------+

尝试写了下但是不对，数据库是 pg 和 mariadb，不知道如何限制每个节点的前两个。

WITH top AS (SELECT * FROM tree WHERE parent=0 LIMIT 2)
(SELECT * FROM top)
UNION
SELECT B.* FROM top 
JOIN (SELECT * FROM tree LIMIT 2) AS B
ON B.parent=top.id

[tianhei826]

with tmp as
(select id
from a
where parent = 0
and rownum <= 2
order by parent asc)
select a.*
from tmp t, a a
where t.id = a.parent
and rownum <= 2
order by parent desc;