recursive select and join related table - sql

Question

CREATE TABLE IF NOT EXISTS "product_category"(
"id" SERIAL NOT NULL,
"parent_id" integer DEFAULT NULL,
"name" varchar DEFAULT NULL,
PRIMARY KEY ("id")
);
CREATE TABLE IF NOT EXISTS "product"(
"id" SERIAL NOT NULL,
"product_category_id" integer DEFAULT NULL,
PRIMARY KEY ("id")
);
I have two table like above,
the product_category is hierarchy,
and product.product_category_id fk product_category.id.
How to select all product under specific product_category id,
e.g
if input product_category 1,
output -> product:1, product:2
if input product_category 2
output -> product:2
product_category
id | parent_id | name
1 | | parent
2 | 1 | child
3 | 2 | child child
product
id | product_category_id
1 | 1
2 | 3
query
like this ?? but this return only product_category list .... I want product list
WITH RECURSIVE pc AS (
SELECT pc.id AS id
FROM product_category pc
LEFT JOIN product p ON p.product_category_id = pc.id
WHERE id = $1
UNION ALL
SELECT child.id
FROM product_category AS child
LEFT JOIN product p ON p.product_category_id = child.id
JOIN pc ON pc.id = child.parent_id
)
SELECT * FROM product_category WHERE id IN (SELECT * FROM pc)

Answer 1

You should first build up the list of categories, then join that to the products.
WITH RECURSIVE pc AS (
SELECT id
FROM product_category
WHERE id = $id
UNION ALL
SELECT child.id
FROM product_category AS child
JOIN pc ON pc.id = child.parent_id
)
SELECT pr.*
FROM product pr
JOIN pc on pr.product_category_id = pc.id
;

Answer 2

Note:I do not have postgresql installed on my system so I am talking based on concepts. I do not see where you have defined the foreign key constraint ... still I am assuming you have done so. And I have not checked the correctness of the CTE/Common Table Expression (- basically with recursive portion of your SQL). Assuming that CTE is correct -
How is about replacing
SELECT * FROM product_category WHERE id IN (SELECT * FROM pc)
with
SELECT * FROM product WHERE product_category_id IN (SELECT * FROM pc)
The correctness of the CTE is the next thing I am going to check.
So apparently the following should work:
;
WITH RECURSIVE pc AS (
SELECT pc.id AS id
FROM
product_category pc
LEFT JOIN product p
ON p.product_category_id = pc.id
WHERE pc.id = <Your product category id of interest>
UNION ALL
SELECT child.id
FROM
product_category AS child
LEFT JOIN product p
ON p.product_category_id = child.id
JOIN pc
ON pc.id = child.parent_id
)
SELECT id as product_id FROM product WHERE product_category_id
IN (
SELECT * FROM pc
);