FULL OUTER JOIN of two INNER JOINs - sql

Question

I have three tables.
My Table Order has an Id and a Date.
Likewise my table Delivery has an Id and a Date.
My third table Part has an Id, and two foreign keys to Order and Delivery: OrderId, DeliveryId.
I want to create a query that gives me an overview over the count of orders and deliveries per month, like this:
+------+-------+------------+---------------+
| Year | Month | OrderCount | DeliveryCount |
+------+-------+------------+---------------+
| 2021 | 2 | 10 | 12 |
+------+-------+------------+---------------+
| 2021 | 1 | 234 | 213 |
+------+-------+------------+---------------+
| ... | ... | ... | ... |
+------+-------+------------+---------------+
So I created a query that gives me the orders per month:
SELECT
MONTH(o.[Date]) AS [Month],
YEAR(o.[Date]) AS [YEAR],
COUNT(p.Id) AS OrderCount
FROM
Part AS p
INNER JOIN
[Order] AS o ON o.Id = p.OrderId
GROUP BY
MONTH(o.[Date]), YEAR(o.[Date])
and one for the deliveries per month:
SELECT
MONTH(d.[Date]) AS [Month],
YEAR(d.[Date]) AS [YEAR],
COUNT(p.Id) AS DeliveryCount
FROM
Part AS p
INNER JOIN
Delivery AS d ON d.Id = p.DeliveryId
GROUP BY
MONTH(d.[Date]), YEAR(d.[Date])
But now I am struggling with joining them. I think I need a FULL OUTER JOIN, because I need the Month/Year/DeliveryCount when there is no order but deliveries and vice versa.
What I have tried is
SELECT
MONTH(o.Date) AS [Month]
YEAR(o.Date) as [Year]
COUNT(p.Id) as OrderCount
FROM
Part AS p
INNER JOIN
[Order] AS o ON o.Id = p.OrderId
GROUP BY
MONTH(o.Date), Year(o.Date)
FULL OUTER JOIN
(SELECT
MONTH(d.Date) AS [Month]
YEAR(d.Date) as [Year]
COUNT(pp.Id) as DeliveryCount
FROM
Part AS pp
INNER JOIN
Delivery AS d ON d.Id = pp.DeliveryId
GROUP BY
MONTH(d.Date), YEAR(d.Date)) AS d ON d.[Month] = MONTH(o.Date) AND d.[Year] = YEAR(o.Date)
But that's not how FULL OUTER JOINS work.
How do I get a full outer of these two inner joins?
Or is this approach of outer joining two inner joins wrong in the first place?

Answer 1

You have to treat your two queries as sub-queries.
This should give you the desired result
Select Orders.OrderCount, Deliveries.DeliveryCount
from (
SELECT
MONTH(o.Date) AS [Month]
YEAR(o.Date) AS [YEAR]
COUNT(p.Id) AS OrderCount
FROM
Part AS p
INNER JOIN
[Order] AS o ON o.Id = p.OrderId
GROUP BY
MONTH(o.Date), YEAR(o.Date)
) Orders
FULL OUTER JOIN
(
SELECT
MONTH(d.Date) AS [Month]
YEAR(d.Date) AS [YEAR]
COUNT(p.Id) AS DeliveryCount
FROM
Part AS p
INNER JOIN
Delivery AS d ON d.Id = p.DeliveryId
GROUP BY
MONTH(d.Date), YEAR(d.Date) ) Deliveries
on Orders.Month = Deliveries.Month
and Orders.Year = Deliveries.Year

Answer 2

This would solve your problem.
In case you have months/year missing, use other join.
WITH OrdersMonth AS (
SELECT
MONTH(o.Date) AS [Month]
YEAR(o.Date) AS [YEAR]
COUNT(p.Id) AS OrderCount
FROM
Part AS p
INNER JOIN
[Order] AS o ON o.Id = p.OrderId
GROUP BY
MONTH(o.Date), YEAR(o.Date)
), DeliveriesMonth AS (
SELECT
MONTH(d.Date) AS [Month]
YEAR(d.Date) AS [YEAR]
COUNT(p.Id) AS DeliveryCount
FROM
Part AS p
INNER JOIN
Delivery AS d ON d.Id = p.DeliveryId
GROUP BY
MONTH(d.Date), YEAR(d.Date)
)
SELECT om.YEAR
,om.Month
,om.OrderCount
,dm.DeliveryCount
FROM OrdersMonth om
LEFT JOIN DeliveriesMonth dm ON om.om.YEAR = dm.YEAR AND om.Month = dm.Month