组从RECURSIVE查询中检索记录，这些记录具有相同的父母，祖父母

Question

我有一个Item（s）表，而不是ManyToMany与Category的关系。

项目

id  |  name 
 1     name1
 2     name2
 3     name3 

物品类别

id | category_id | item_id
  1     4            1
  2     5            1
  3     7            1
  4     8            1

类别（parent_id自身的外键）

  id | parent_id | name     
   1     Null       A1
   2      1         B1
   3      1         B2 
   4      2         C1
   5      3         C2
   6      1         D1
   7      6         DE
   8      1         DT

我正在尝试获取项目类别，从子项到父项目，因此我使用：

 WITH RECURSIVE descendants(name, id, slug, parent_id, bread_order) AS (
 SELECT  name, id, slug, parent_id, 0
 FROM categories
 where id in (
 SELECT c.id FROM items AS p
 INNER JOIN items_categories AS pc ON p.id=pc.item_id 
 INNER JOIN categories AS c ON pc.category_id = c.id
     WHERE p.id = 10
 )   
 UNION ALL
 SELECT c2.name, c2.id, c2.slug, c2.parent_id, bread_order+1
 FROM categories AS c2
 INNER JOIN descendants AS d ON c2.id=d.parent_id
) SELECT id, name, parent_id, bread_order FROM descendants 
ORDER by bread_order DESC

问题是两个子类别可以具有相同的父级，或者一个父级是另一个的父级。

C1 -> B1 -> A1
C2 ->B1  -> A1
C3 ->B2  -> A1
DE -> D1 -> A1
DT  -> A1

我试图将它们分组但是，因为有相同的父母，祖父母不是我需要的，当我从数据库中检索信息以了解如何制作路径时我需要（参见上文）。

有没有办法，比如使用带有从子查询返回的ID的CASE？

Answer 1

您可以在查询期间聚合路径：

WITH RECURSIVE descendants(name, id, parent_id, bread_order, path) AS (
  SELECT  name, id, parent_id, 0, array[id] as path
  FROM categories
  where id in (SELECT c.id
               FROM items AS p
                 JOIN items_categories AS pc ON p.id=pc.item_id 
                 JOIN categories AS c ON pc.category_id = c.id
               WHERE p.id = 1)   
  UNION ALL

  SELECT c2.name, c2.id, c2.parent_id, bread_order+1, d.path||c2.id
  FROM categories AS c2
    JOIN descendants AS d ON c2.id=d.parent_id
) 
SELECT id, name, parent_id, path, bread_order
FROM descendants 
ORDER by path