SQL -- Counting with nested subquery - mysql

Question

I have a collection of tables in a relational database
products
categories
orders
line_items
customers
Products has a many-to-many relationship with categories (join table categories_products) and also has and belongs to many orders through line_items, which is a join table for products and orders with an id. A customer also has many orders.
I'm trying to put together some SQL that will give me this sort of response:
customer_id | customer_first_name | category_id | category_name | number_purchased
-----------------------------------
1 |Jack | 1 | Electronics | 15
2 |Jill | 1 | Electronics | 2
2 |Jill | 2 | Hiking | 3
This is the giant hunk of SQL I've been trying to use to get these values:
SELECT
DISTINCT customers.id AS customer_id,
customers.first_name AS customer_first_name,
categories.id AS category_id,
categories.name AS category_name,
(
SELECT count(li.id) FROM line_items li
INNER JOIN orders o ON li.order_id = o.id
INNER JOIN products p ON li.product_id = p.id
INNER JOIN categories_products cp ON cp.product_id = p.id
WHERE
o.customer_id = customer_id
AND o.status = 3
AND cp.category_id = category_id
) AS number_purchased
FROM orders
LEFT JOIN customers ON orders.customer_id = customers.id
LEFT JOIN line_items li ON li.order_id = orders.id
LEFT JOIN products ON products.id = li.product_id
LEFT JOIN categories_products catpr ON catpr.product_id = products.id
LEFT JOIN categories ON catpr.category_id = categories.id
Only the count itself is wrong. Instead of getting the number of line items a customer has bought in a specific category, I'm instead getting a count for all LineItems that have been part of a completed order.
How can I get the count to correctly represent the number of line_items purchased by a specific customer within a category?
NOTE: in the SQL text, o.status = 3 is using an enum to indicate that an Order is "complete."

Answer 1

I think your inner join with categories_products is screwing this up. You should set up a fiddle, like #Strawberry suggested, or try this:
SELECT
DISTINCT customers.id AS customer_id,
customers.first_name AS customer_first_name,
categories.id AS category_id,
categories.name AS category_name,
(
SELECT count(li.id) FROM line_items li
INNER JOIN orders o ON li.order_id = o.id
INNER JOIN products p ON li.product_id = p.id
WHERE
o.customer_id = customer_id
AND o.status = 3
) AS number_purchased
FROM orders
LEFT JOIN customers ON orders.customer_id = customers.id
LEFT JOIN line_items li ON li.order_id = orders.id
LEFT JOIN products ON products.id = li.product_id
LEFT JOIN categories_products catpr ON catpr.product_id = products.id
LEFT JOIN categories ON catpr.category_id = categories.id

Answer 2

If you wanted to correct your count, I would advise using a GROUP BY clause in the subquery. If you GROUP BY orders then you will only get the specific order which you retrieved when looking that the user id was correct. I would encourage you to take a look at mistakes in other part of your SQL code to clean up this hulking query. For example, make sure you want to be using distinct and that you actually want to be using left joins versus inner joins, both of which could seriously mess with the performance of your program.