Subtract two SUM GROUP BY fields - sql

Question

I have two tables itemOrders and itemUsage.
itemOrders has two fields: item_number and qty_ordered
itemUsage has two fields: item_number and qty_used
I'm trying to word my SQL query so that it sums up the quantity of each item number in both tables then subtracts the totals in itemUsage from itemOrders
I've come up with this so far:
SELECT itemOrders.item_number
,(
SELECT sum(qty_ordered)
FROM itemOrders
GROUP BY itemOrders.item_number
) - (
SELECT sum(qty_used)
FROM itemUsage
GROUP BY itemUsage.item_number
) AS item_total
FROM itemOrders
INNER JOIN itemUsage
ON itemOrders.item_number = itemUsage.item_number
GROUP BY itemOrders.item_number
What happens here is that all fields come out to 0.
Example if item number "A" was showing a total quantity of 3 ordered across all instances of "A" in the itemOrders table, and only a total quantity used of 1 across all instances of "A" in the itemUsage table. The sql should show the number one in the item_total field next to 1 instance of "A" in the item_number field.

Answer 1

The problem is you are creating a CARTESIAN PRODUCT and repeting the values on the SUM just calculate each value separated and then LEFT JOIN both. In case no item are used COALESCE will convert NULL to 0
SELECT io_total.item_number,
order_total - COALESCE(used_total, 0) as item_total
FROM (SELECT io.item_number, sum(io.qty_ordered) as order_total
FROM itemOrders io
GROUP BY io.item_number
) io_total
LEFT JOIN (SELECT iu.item_number, sum(iu.qty_used) as used_total
FROM itemUsage iu
GROUP BY iu.item_number
) as iutotal
ON io_total.item_number = iutotal.item_number

Answer 2

Well it looks like you are making three queries, two separate ones, one for each sum, and one with an inner join that isn't being used.
Try
Select itemOrders.item_number, sum(itemOrders.qty_ordered - itemUsage.qty_used) as item_total
from itemOrders INNER JOIN itemUsage
On itemOrders.item_number = itemUsage.item_number
GROUP BY itemOrders.item_number