我正在使用 android SQLite 数据库,结果如下:
id | root_id | parent_id |name
----------------------------------
613 | null | null | m1
612 | null | null | m4
570 | null | null | m1
635 | 570 | 570 | m6
653 | 570 | 635 | m1
652 | 570 | 635 | m3
632 | 570 | 570 | m9
392 | null | null | m2
753 | 392 | 392 | m5
751 | 392 | 392 | m4
391 | null | null | m7
我通过以下查询得到了这个结果:
WITH RECURSIVE all_employees(id, root_id, parent_id, name) AS (
SELECT id, id AS root_id, parent_id, name FROM employees WHERE parent_id IS NULL
UNION ALL
SELECT c.id, (CASE WHEN c.parent_id IS NOT NULL THEN p.root_id END), c.parent_id, c.name FROM employees c JOIN all_employees p ON p.id = c.parent_id ORDER BY id DESC
)
SELECT id, (CASE WHEN id != root_id THEN root_id ELSE NULL END) root_id, parent_id, name FROM all_employees
我想为具有相同 root_id 的重复行设置限制,例如: 加载 root_id 570 的 2 行和 root_id 392 的 1 行:
id | root_id | parent_id |name
----------------------------------
613 | null | null | m1
612 | null | null | m4
570 | null | null | m1
635 | 570 | 570 | m6
653 | 570 | 635 | m1
392 | null | null | m2
753 | 392 | 392 | m5
391 | null | null | m7
最佳答案
创建另一个CTE
:
counters(root_id, n) AS (VALUES (570, 2), (392, 1))
在其中返回您想要限制其行数的所有 root_id
以及每个 root_id 的行数,然后使用递归的
到此。LEFT
连接>CTE
最后,使用相关子查询在 WHERE
子句中设置条件:
WITH
RECURSIVE all_employees(id, root_id, parent_id, name) AS (
SELECT id, id AS root_id, parent_id, name
FROM employees
WHERE parent_id IS NULL
UNION ALL
SELECT c.id, (CASE WHEN c.parent_id IS NOT NULL THEN p.root_id END), c.parent_id, c.name
FROM employees c JOIN all_employees p
ON p.id = c.parent_id
),
counters(root_id, n) AS (VALUES (570, 2), (392, 1))
SELECT a.id, (CASE WHEN a.id <> a.root_id THEN a.root_id END) root_id, a.parent_id, a.name
FROM all_employees a LEFT JOIN counters c
ON c.root_id = a.root_id
WHERE c.root_id IS NULL
OR (SELECT COUNT(*) FROM all_employees b WHERE b.root_id IS a.root_id AND b.id <= a.id) <= c.n
请注意,CTE
中的 ORDER BY
子句是无用的,因为当您从 中选择时,不能保证按该顺序返回行。 >CTE
。
您可以设置最终查询中所需的行顺序。
关于android - Sqlite限制具有相同id的重复行,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/68343060/