Sqlite限制重复行具有相同的ID

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的重复行设置一个限制，例如：使用 root_id 570 加载 2 行，使用 root_id 392 加载 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

Answer 1

有限选择联合

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, root_id, parent_id, name
FROM all_employees
where root_id not in (570, 392)
union all
select *
from (
   select *
   from all_employees
   where root_id =570 
   limit 3
)
union all
select *
from (
   select *
   from all_employees
   where root_id =392
   limit 2
)

Answer 2

创建另一个 CTE:

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

你 return 所有 root_id 想要限制它们的行和每个行的行数然后使用递归 [=] 的 LEFT 连接12=] 到那个。
最后，在 WHERE 子句中设置带有相关子查询的条件：

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 select来自 CTE.
您可以在最终查询中设置所需行的顺序。

Answer 3

向 CTE 添加计数器，然后使用 WHERE 逻辑进行过滤：

WITH RECURSIVE all_employees(id, root_id, parent_id, name, lev) AS (
    SELECT id, id AS root_id, parent_id, name, 0 as lev
    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, p.lev + 1
    FROM employees c JOIN
         all_employees p
         ON p.id = c.parent_id
    ORDER BY id DESC   -- don't think this does anything
   )
SELECT id, (CASE WHEN id <> root_id THEN root_id END) as root_id, parent_id, name
FROM all_employees
WHERE root_id = 570 AND cnt <= 1 OR
      root_id = 392 AND cnt <= 2 OR
      root_id NOT IN (570, 392);

这里的重点是将计数放入递归 CTE，然后使用该信息进行过滤。您可以根据自己的喜好对 WHERE 子句进行措辞：

WHERE (CASE WHEN root_id = 570 THEN cnt <= 1
            WHEN root_id = 392 THEN cnt <= 2
            ELSE 1
       END)

Sqlite限制重复行具有相同的ID

Sqlite limit repeated rows with same id

database

sqlite

android

common-table-expression

android-sqlite