PostgreSQL的 - 計算次數層次結構中的元素

Question

有一個表：

CREATE TABLE product_categories (
id INT NOT NULL PRIMARY KEY, 
parent INT NOT NULL, 
name varchar(50) NOT NULL, 
isProduct boolean NOT NULL 
); 

有沒有辦法在每個類別計算數量的產品？

即：

INSERT INTO product_categories VALUES (1, NULL, 'Main', 'no'); 

INSERT INTO product_categories VALUES (2, 1, 'Plant', 'no'); 
INSERT INTO product_categories VALUES (3, 2, 'Cactus', 'yes'); 
INSERT INTO product_categories VALUES (4, 2, 'Spruce', 'yes'); 
INSERT INTO product_categories VALUES (5, 2, 'Birch', 'yes'); 
INSERT INTO product_categories VALUES (6, 2, 'Pine', 'yes'); 

INSERT INTO product_categories VALUES (7, 1, 'Stock', 'no'); 
INSERT INTO product_categories VALUES (8, 7, 'Spade', 'yes'); 
INSERT INTO product_categories VALUES (9, 7, 'Watering can', 'yes'); 

，並應接受：

Category | Count 
Main  | 6 
Plant | 4 
Stock | 2 

Answer 1

您需要使用Recursive Common Table Expression

WITH RECURSIVE Parents AS 
( SELECT ID, Parent, Name, IsProduct 
    FROM product_categories 
    WHERE Parent IS NOT NULL 
    UNION ALL 
    SELECT c.ID, p.Parent, c.Name, c.IsProduct 
    FROM product_categories c 
      INNER JOIN Parents p 
       ON p.ID = c.Parent 
) 
SELECT pc.Name, 
     COUNT(*) AS Products, 
     ARRAY_AGG(p.Name) AS ProductList 
FROM product_categories pc 
     INNER JOIN Parents p 
      ON p.Parent = pc.ID 
WHERE p.IsProduct = 'yes' 
GROUP BY pc.Name 

Working Example