How to select unique value from many to many mysql relationship - php

Question

Hi I got confused in this sql case. I am using mysql Ver 15.1 Distrib 10.2.31-MariaDB. And let's say I have 3 table products, categories, and product_categories as pivot table.
Here is the data example:
products:
| id | name |
------------------------
| 1 | asd wef |
| 2 | gggg2222 |
| 3 | pppga 99 |
| 4 | lalala 55 |
And for categories:
| id | level | parent_id | name |
-----------------------------------
| 20 | 1 | | Fashion |
| 22 | 2 | 20 | Top |
| 23 | 3 | 22 | T-Shirt |
| 24 | 3 | 22 | Jacket |
And for the pivot table, product_categories:
| product_id | category_id |
--------------------------------
| 1 | 20 |
| 1 | 22 |
| 1 | 23 |
| 2 | 22 |
| 2 | 20 |
| 3 | 20 |
| 4 | 20 |
So as you can see from pivot table only product_id = 3 & 4 that stop in category level 1. And product_id = 2 only stop in category level 2.
What I would like to achieve here is when I select from categories table. I can count how many product that stopping here. This is example of the data that I want to get.
"categories": [
{
"id": 20,
"total_product": 4
"stopped_product": 2
},
{
"id": 22,
"total_product": 3
"stopped_product": 1
}
]
So far I tried using group by:
SELECT * FROM product_categories WHERE product_id IN (1, 2, 3, 4) GROUP BY product_id HAVING category_id=20
output:
| product_id | category_id |
--------------------------------
| 1 | 20 |
| 3 | 20 |
| 4 | 20 |
Expected Output
| product_id | category_id |
--------------------------------
| 3 | 20 |
| 4 | 20 |

Answer 1

You can use a CTE to generate a list of stop categories for each product and then JOIN that to the categories and product_categories tables to count the total number of products and number of stopped products for each category:
WITH prods AS (
SELECT product_id, MAX(category_id) AS stop_cat
FROM product_categories
GROUP BY product_id
)
SELECT c.id AS category_id,
COUNT(DISTINCT pc.product_id) AS total_product,
SUM(c.id = p.stop_cat) AS stopped_product
FROM categories c
JOIN product_categories pc ON pc.category_id = c.id
JOIN prods p ON p.product_id = pc.product_id
GROUP BY c.id
Output (for your sample data)
category_id total_product stopped_product
20 4 2
22 2 1
23 1 1
Demo on dbfiddle

Answer 2

I guess you want max values only
SELECT product_id,MAX(category_id) as category_id FROM product_categories WHERE product_id IN (1, 2,
3, 4)
GROUP BY product_id
HAVING category_id=20

Answer 3

Here is you can find category-wise product count and ids:
SELECT c.name as categoryName, c.level, COUNT(pc.product_id) as productCount, GROUP_CONCAT(pc.product_id) as productIds
FROM product_categories pc JOIN categories c ON c.id = pc.category_id
WHERE 1
GROUP by c.id