SQL bestsellers query - sql

Question

I have a problem with finding bestselling books in each category. With attached code I can find only categories of needed books but how to find this books?
SELECT b.category_id, max(b.total_quantity) as max_quantity
FROM (
SELECT books.id, books.category_id, sum(order_items.quantity) as total_quantity
FROM order_items
INNER JOIN orders ON order_items.order_id = orders.id
INNER JOIN books on order_items.book_id = books.id
WHERE orders.status in (2, 3)
GROUP BY books.id
) as b
GROUP BY b.category_id

Answer 1

Use distinct on:
SELECT DISTINCT ON (b.id), b.*, SUM(oi.quantity) as total_quantity
FROM order_items oi JOIN
orders o
ON oi.order_id = o.id JOIN
books b
ON oi.book_id = b.id
WHERE o.status in (2, 3)
GROUP BY b.id
ORDER BY b.category_id, total_quantity DESC

Answer 2

You could filter in the having clause with a correlated, aggregate query that returns the top selling quantity for the related category:
select b.id, b.category_id, sum(oi.quantity) as total_quantity
from order_items oi
inner join orders o on oi.order_id = o.id and o.status in (2, 3)
inner join books b on oi.book_id = b.id
having sum(oi.quantity) = (
select sum(oi.quantity)
from order_items oi1
inner join orders o1 on oi1.order_id = o1.id and o1.status in (2, 3)
inner join books b1 on oi1.book_id = b1.id
where b1.category_id = b.category_id
group by b1.id
order by sum(oi.quantity) desc
limit 1
)
group by b.id, b.category_id
Or, if your RDBMS supports window functions (and allows mixing them with aggregation):
select id, category_id, total_quantity
from (
select
b.id,
b.category_id,
sum(oi.quantity) as total_quantity,
rank() over(partition by b.category_id order by sum(oi.quantity) desc) rn
from order_items oi
inner join orders o on oi.order_id = o.id and o.status in (2, 3)
inner join books b on oi.book_id = b.id
group by b.id, b.category_id
) t
where rn = 1

Answer 3

I would use a DENSE_RANK over the SUM.
If available in the RDBMS.
SELECT *
FROM
(
SELECT books.id, books.category_id
, SUM(orditm.quantity) AS total_quantity
, DENSE_RANK() OVER (PARTITION BY books.category_id ORDER BY SUM(order_items.quantity) DESC) AS Rnk
FROM order_items AS orditm
JOIN orders ON orditm.order_id = orders.id
JOIN books ON orditm.book_id = books.id
WHERE orders.status IN (2, 3)
GROUP BY books.id, books.category_id
) q
WHERE Rnk = 1

Answer 4

I guess you must have book_name column in your books table. So you can try below query -
SELECT b.book_name, b.category_id, max(b.total_quantity) as max_quantity
FROM (
SELECT books.id, books.book_name, books.category_id, sum(order_items.quantity) as total_quantity
FROM order_items
INNER JOIN orders ON order_items.order_id = orders.id
INNER JOIN books on order_items.book_id = books.id
WHERE orders.status in (2, 3)
GROUP BY books.id
) as b
GROUP BY b.category_id