How to correctly separate this query - mysql

Question

I'm trying to sort through my table to find the frequent categories in my orders. After conducting this query
SELECT
ccd.cart_id,
mp.category_name,
ccd.quantity
FROM
customer_orders co
JOIN customer_cart_dtls ccd
ON co.order_cart = ccd.cart_id
JOIN merchant_products mp
ON ccd.product_id = mp.product_id
which yields this result
So from that query Cart #2006........63 has 9 items. 1 from eatables, 3 From fruits, 2 From cleaning, and 3 from Snacks. All of them quantity 1 except for the second entry of cleaning which has two. How can I alter my query so that I get 10 items all with quantity 1?
Which would look like this

Answer 1

You want to split the individual rows into multiple rows. One method uses recursive CTEs:
WITH RECURSIVE t as (
SELECT ccd.cart_id, mp.category_name, ccd.quantity
FROM customer_orders co JOIN
customer_cart_dtls ccd
ON co.order_cart = ccd.cart_id JOIN
merchant_products mp
ON ccd.product_id = mp.product_id
),
cte as (
SELECT cart_id, category_name, quantity, 1 as n
FROM t
UNION ALL
SELECT cart_id, category_name, quantity, n + 1
FROM cte
WHERE n < quantity
)
SELECT cart_id, category_name, 1 as quantity
FROM cte;
Here is a db<>fiddle.
EDIT:
You can join in a list of quantities -- easier if you have a tally table of some sort:
SELECT ccd.cart_id, mp.category_name, 1 as quantity
FROM customer_orders co JOIN
customer_cart_dtls ccd
ON co.order_cart = ccd.cart_id JOIN
merchant_products mp
ON ccd.product_id = mp.product_id JOIN
(SELECT 1 as n UNION ALL
SELECT 2 as n UNION ALL
SELECT 3 as n UNION ALL
SELECT 4 as n UNION ALL
SELECT 5 as n
) n
ON n.n <= ccd.quantity;
You can also construct the table using variables from an existing table (if it is big enough):
(select (#rn := #rn + 1) as n
from customer_orders cross join
(select #rn := 0) params
limit 100 -- say that 100 is big enough
) n

Answer 2

Are you trying to count how many items come from each category by splitting every item into an individual row and then using COUNT? If so, I don't think you necessarily need to go down that route. It will likely be a lot easier to simply use the SUM aggregate function after grouping by category_name. It might look something like this:
SELECT mp.category_name, SUM(ccd.quantity)
FROM customer_orders AS co
JOIN customer_card_dtls AS ccd ON co.order_cart = ccd.cart_id
JOIN merchant_products AS mp ON ccd.product_id = mp.product_id
GROUP BY mp.category_name
If you want to also see cart IDs then just add the appropriate columns to your SELECT and GROUP BY statements