Way To Include New Warehouses / Products In Query

I have to do a number of database exercises and it's my very first time using SQL. I did all of them, except for one query.It's fairly simple, I need to find for every combination of warehouses (there are 5) and items (25) how many of each items I can stock in said warehouse until until it goes over the capacity. So for example warehouse 1 has a max of 300 pounds and item 1 weighs 1 pound, so I can stock 300 in it. Warehouse 1 and product 2...etc.

Every warehouse has a capacity in pounds and every item has a weight also in pounds. There are 3 tables between warehouse and items (warehouse---shipment---order----order details----items).

I managed to find every instance of warehouse_ID/item_ID/(capacity/weight)...but thats for the existing warehouses/item. If they added a new warehouse or product, I'd be screwed, as they wouldnt show up in my query...here's my query:

select Warehouse_ID, item_ID, (select round(capacity/weight,1) as maximum_capacity
from order_detail
join item using (item_ID)
join order using (Order_ID)
join shipment using (shipment_number)
join warehouse using (warehouse_ID)
group by warehouse_id, item_id
order by warehouse_ID

I keep trying to find a way to include possible new warehouses or products in my query, but I'm at a complete loss. I tried to right and left outer joins with no luck