选择所有产品并通过子类别（未知级别）加入主要类别

Question

我有2张桌子

分类

id - name - parent
1 - Category A - 0
2 - Category B - 0
3 - Category C - 0
4 - Category D - 0
5 - Subcategory Of 1 - 1
6 - Subcategory Of 5 - 5
7 - Subcategory Of 5 - 5

产品

id - name - category - description
1  - Name - 5 - Description

如何选择所有产品并通过子类别加入主要类别？产品类别只能有1或2或3或4个级别（未知级别）。

我使用＆＃34; WITH RECURSIVE＆＃34;在类别表中，但无法找到将产品表与1次查询相结合的方法

WITH RECURSIVE category_child AS 
(
    SELECT * FROM categories as c WHERE c.id = 5
    UNION ALL
    SELECT c2.* FROM categories AS c2
    JOIN category_child as c3 ON c3.parent_id = c2.id
)

最好的方法是什么？

预期结果

id - name - category - description - root - sub category id 1 - sub category id 2 - sub category id 3

OR

id - name - category - description - root
id - name - category - description - sub category id 1
id - name - category - description - sub category id 2
id - name - category - description - sub category id 3 

Answer 1

如果您想要一个类别的完整路径，则无法使用c.id = 5启动非递归部分，您必须使用where parent_id is null从根开始（您应该不< / strong>使用不存在的类别ID标识根节点，这会阻止为parent_id列创建正确的外键。）

在递归部分，您可以聚合根类别的完整路径：

with recursive tree as 
(
  select *, id as root_category, concat('/', name) as category_path
  from category
  where parent_id is null
  union all
  select c.*, p.root_category, concat(p.category_path, '/', c.name)
  from category c
    join tree p on c.parent_id = p.id
)
select p.id as product_id,
       p.name as product_name,
       t.root_category,
       t.category_path
from tree t
  join product p on p.category = t.id

使用以下示例数据：

create table category (id integer, name text, parent_id integer);
create table product (id integer, name text, category integer, description text);

insert into category
values
(1, 'Category A', null),
(2, 'Category B', null),
(3, 'Category C', null),
(4, 'Category D', null),
(5, 'Subcategory Of 1', 1),
(6, 'Subcategory Of 5', 5),
(7, 'Subcategory Of 5', 5),
(8, 'Subcategory of D', 4)
;

insert into product
values
(1, 'Product One', 5, 'Our first product'),
(2, 'Product Two', 8, 'The even better one');

返回：

product_id | product_name | root_category | category_path               
-----------+--------------+---------------+-----------------------------
         1 | Product One  |             1 | /Category A/Subcategory Of 1
         2 | Product Two  |             4 | /Category D/Subcategory of D