Join 3 tables, Create new Row if Unique - sql

Question

I have three tables that I'm trying to join.
I join the first two:
Purchase Orders
+-----+----------+
| ID | location |
+-----+----------+
| 1 | Canada |
| 2 | USA |
+-----+----------+
Stock
+-----+----------+----------------+
| SKU | OnOrder |PurchaseOrderID |
+-----+----------+----------------+
| ABC | 30 |2 |
| DEF | 40 |1 |
+-----+----------+----------------+
And my result is:
+-----+----------+----------------+
| SKU | OnOrder |location |
+-----+----------+----------------+
| ABC | 30 |USA |
| DEF | 40 |Canada |
+-----+----------+----------------+
And now I want to join this result with another table as shown below
ItemOrders
+-----+----------+----------------+
| SKU | Quantity |Location |
+-----+----------+----------------+
| ABC | 88 |USA |
| DEF | 99 |Mexico |
+-----+----------+----------------+
Where:
location of Join1 equals location of Table 3, and
SKU of Join 1 equals Table 3 .
If the match doesn't exist, it should make a new row
So I want my output to be:
+-----+----------+--------------------------+
| SKU | OnOrder |Quantity |Location |
+-----+----------+---------+----------------+
| ABC | 30 |88 |USA |
| DEF | 40 | |Canada |
| DEF | |99 |Mexico |
+-----+----------+---------+----------------+
I really can't wrap my head around this for some reason. I've tried two joins, a merge, two joins and a union. I'm trying to do this in laravel FWIW.
Your help is much appreciated!

Answer 1

I think you would create 2 queries where both return the same columns and then UNION them together.
Wrap that entire thing in a subquery and GROUP BY SKU, Location and use an aggregate function as a way to aggregate the 2 rows into one.
Aggregate functions usually ignore Null values so simply a MAX() might work.
Like,
SELECT SKU, Location, max(OnOrder) as OnOrder, max(Quantity) as Quantity
FROM
(
SELECT SKU, OnOrder, NULL as Quantity, Location
FROM {stuff to build output 1}
UNION ALL
SELECT SKU, NULL as OnOrder, Quantity, Location
FROM {stuff to build output 2}
) rs
GROUP BY SKU, Location;