SQL-Group by ID´s from tables - sql

Question

So i have 2 tables let's say
Orders(order_id,etc...) and
order_items(order_id,...)
I want to get an overview of how many orders have x amount of items by grouping the id from orders with the x amount of ids from order items
So lets say atm I have this scenario:
orders:
order_id
???
???
11
1
Pending
33
3
Pending
44
3
Pending
22
2
Pending
444
4
Pending
order_items:
order_id
???
???
???
???
11
7
32
132
469,99
22
5
192
124
519,99
22
7
27
92
800,74
33
11
6
128
849,99
33
10
95
106
109,99
33
5
271
148
549,59
44
9
284
138
54,99
44
8
174
117
798,26
44
6
131
34
279,99
44
11
271
58
549,59
444
9
284
138
54,99
444
8
174
117
798,26
444
6
131
34
279,99
444
11
271
58
549,59
With this query:
SELECT count(o.order_id) as ORDER_count,
(SELECT count(*)
FROM order_items ol
WHERE o.order_id = ol.order_id) AS ORDER_ITEMS_count
FROM orders o, order_items ol
WHERE o.order_id = ol.order_id
GROUP BY o.order_id,ol.order_id;
I'm getting:
ORDER_COUNT
ORDER_ITEMS_COUNT
4
4
1
1
2
2
3
3
4
4
and i'm expecting something like:
ORDER_COUNT
ORDER_ITEMS_COUNT
5
5
1
1
2
2
3
3
Can you please point out what am I missing?
Thankx for the help

Answer 1

I want to get an overview of how many orders have x amount of items by grouping the id from orders with the x amount of ids from order items
First select all orders and count their items. Then use this result to count how many orders share a number of items.
SELECT number_of_items, COUNT(*) AS how_many_orders
FROM
(
SELECT o.order_id, COUNT(oi.order_id) as number_of_items
FROM orders o
LEFT OUTER JOIN order_items ON oi.order_id = o.order_id
GROUP BY o.order_id
) orders_with_item_counts
GROUP BY number_of_items
ORDER BY number_of_items;
If every order has at least one item (which seems likely), you can change the LEFT OUTER JOIN to an INNER JOIN.