使用一个语句的输出作为另一个语句的输入（递归）

Question

product 表通过product.ext_category_id=categories.ext_id 与categories 表相关联，但是ext_category_id 可能与categories 表中的1 级、2 级或3 级类别相关联。

我正在寻找一个 select 语句，它将选择 products 表中的所有产品以及从类别表中相应的级别 1 类别。

product.id	product.product_name	categories.name
1	Strawberries	Fruit & Vegetable department
2	Bananas	Fruit & Vegetable department
3	Potatoes	Fruit & Vegetable department
4	Car Battery	Automotive department
5	Chips	Junk Food department

如果products 表中的产品与categories 表中的2 级或3 级类别相关联，则您需要使用categories.parent_id 搜索@987654334 中的categories.id 列@table 查找下一个类别，直到获得第 1 级类别。

我花了一天时间尝试使用各种方法来解决这个问题，例如 CASE 和带有 select 语句的子查询以及递归函数，但没有任何效果。示例 CASE sn-p -

SELECT
    product.id,
    product.product_name,
    CASE WHEN categories.level = '3' THEN (
           SELECT c2.name
           FROM categories c2
           WHERE c2.id = categories.parent_id )
    END AS level_2_cat_name
FROM product
    JOIN categories ON categories.ext_id = product.ext_category_id

示例product 表

id	product_name	ext_category_id
1	Strawberries	101
2	Bananas	102
3	Potatoes	103
4	Car Battery	104
5	Chips	105

示例categories 表

id	ext_id	name	level	parent_id
1001	101	Fruit	2	2000
1002	102	Fruit	2	2000
1003	103	Vegetables	2	2000
1004	104	Car Parts	3	2500
2001	209	Junk Food Department	1	Null

...

id	ext_id	name	level	parent_id
2000	205	Fruit & Vegetable department	1	Null
2000	205	Fruit & Vegetable department	1	Null
2000	205	Fruit & Vegetable department	1	Null
2500	309	Cars & Trucks	2	2002
2002	209	Automotive department	1	Null
2001	209	Junk Food department	1	Null

Answer 1

这是一个使用递归 CTE 解决问题的示例。 这几乎可以处理任意数量的类别级别。

有几种方法可以做到这一点。

您的测试数据有一些错误。我已经为这次测试更正了它们。

注意：存在外连接，因为您的初始数据没有与 Chips 相关的匹配类别。如果没有丢失的数据，可以将其删除。我现在要删除它。

The fiddle

WITH RECURSIVE cte1 (id, name, cid, cname, level, parent_id) AS (
        SELECT p.id, p.product_name, c.id, c.name, level, parent_id
          FROM product     AS p
          JOIN categories  AS c
            ON p.ext_category_id = c.ext_id
         UNION ALL
        SELECT p.id, p.name, c.id AS cid, c.name AS cname, c.level, c.parent_id
          FROM cte1        AS p
          JOIN categories  AS c
            ON c.id = p.parent_id
     )
SELECT id, name, cname
  FROM cte1
 WHERE level = 1
 ORDER BY id
;

结果：

+------+--------------+------------------------------+
| id   | name         | cname                        |
+------+--------------+------------------------------+
|    1 | Strawberries | Fruit & Vegetable department |
|    2 | Bananas      | Fruit & Vegetable department |
|    3 | Potatoes     | Fruit & Vegetable department |
|    4 | Car Battery  | Automotive department        |
|    5 | Chips        | Junk Food Department         |
+------+--------------+------------------------------+

设置：

CREATE TABLE product (
    id               int
  , product_name     varchar(40)
  , ext_category_id  int
);

INSERT INTO product VALUES
  ( 1, 'Strawberries' , 101)
, ( 2, 'Bananas'      , 102)
, ( 3, 'Potatoes'     , 103)
, ( 4, 'Car Battery'  , 104)
, ( 5, 'Chips'        , 105)
;

CREATE TABLE categories (
    id           int
  , ext_id       int
  , name         varchar(40)
  , level        int
  , parent_id    int
);

-- id   ext_id  name    level   parent_id
INSERT INTO categories VALUES
  (1001, 101, 'Fruit'                , 2, 2000)
, (1002, 102, 'Fruit'                , 2, 2000)
, (1003, 103, 'Vegetables'           , 2, 2000)
, (1004, 104, 'Car Parts'            , 3, 2500)
, (1005, 105, 'FoodParts'            , 2, 2001)
, (2001, 209, 'Junk Food Department' , 1, Null)
, (2000, 205, 'Fruit & Vegetable department', 1, Null)
, (2500, 309, 'Cars & Trucks'               , 2, 2002)
, (2002, 209, 'Automotive department'       , 1, Null)
;