android - Sqlite限制具有相同ID的重复行

Question

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

我通过下面的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 2 行 root_id 570 和 1 行 root_id 392:

 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

Answer 1

创建另一个 CTE :

counters(root_id, n) AS (VALUES (570, 2), (392, 1)) 

您返回所有 root_id 的地方s 你想限制他们的行和每一行的行数，然后使用 LEFT递归的加入 CTE到那个。
最后，在 WHERE中设置条件带有相关子查询的子句:

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 ORDER BY CTE内的条款没用，因为当您从 CTE 中选择时，不能保证行会按该顺序返回。 .
您可以在最终查询中设置所需行的顺序。