Subtracting results from two queries / sub queries - sql

Question

I'm trying to create a query which returns the results of customer orders (created in a month e.g. January) - the cancelled customer orders in that exact month (cancelled customer orders in month of January) and display the results grouped by location (Rows) and by year with month (Columns).
Currently I have a table containing all the customer order information both created and cancelled. Each customer order has a unique order number, location where it was sold, creation date and cancellation date. If the customer order is still valid, then the cancellation date will be null or "//". If the customer order is cancelled then it will have a cancellation date. As some additional information a customer order can be created in January 2019 and cancelled in July or August, or December etc. What I would like to obtain is the net customer orders for all the months by doing gross customer orders for a month - cancelled customer orders for that month and for a specific location = net customer orders for that month for that location.
In order to achieve this what I have tried, was to create two separate queries from the table, first one containing all the valid customer orders and the second one containing all the cancellations. Then i tried creating a cross-tab between the two other queries, trying to count what I mentioned above, grouping by location and then pivoting the of the year and month.
First query with valid customer orders named cust_valid (simplified):
SELECT cust_ords.[SaleLoc], cust_ords.[OrderNum], cust_ords.[CreationDate], cust_ords.[CancelDate]
FROM cust_ords
WHERE cust_ords.[CancelDate] = "" OR cust_ords.[CancelDate] = "//";
Second query with cancelled customer orders named cust_cancelled (simplified):
SELECT cust_ords.[SaleLoc], cust_ords.[OrderNum], cust_ords.[CreationDate], cust_ords.[CancelDate]
FROM cust_ords
WHERE cust_ords.[CancelDate] <> "" OR cust_ords.[CancelDate] <> "//";
Last, a cross-tab between them:
TRANSFORM Count(cust_valid.[OrderNum]) AS [NetOrderCount]
SELECT cust_valid.[SaleLoc]
FROM cust_valid LEFT JOIN cust_cancelled ON cust_valid.[CreationDate] = cust_cancelled.[CancelDate]
WHERE cust_valid.[CreationDate] = cust_cancelled.[CancelDate]
GROUP BY cust_ords.[SaleLoc]
PIVOT cust_valid.[CreationDate];
In this sense, I am trying to obtain (count) the net customer orders (total created for a month - what was cancelled in that month) for every given location and display the results per month (basically the columns names should be the year and the month). So for example if i have 10 customer orders in January, 5 in February and 15 in March, if 3 of the ones in January get cancelled in March, then I would like to count for the month of March 15 - 3, thus ending up with January 10, February 5, March 12.

Answer 1

First of all, you say an order is valid is valid if cancellation date is null or //, however you test for:
WHERE cust_ords.[CancelDate] = "" OR cust_ords.[CancelDate] = "//";
To test for null use [CancelDate] is null, or shorthand the test to ISNULL([CancelDate],'//')='//'
Second, in your second query you test for cancelled orders, with
WHERE cust_ords.[CancelDate] <> "" OR cust_ords.[CancelDate] <> "//";
That is not the negation of your test for cancelled orders!
!(A or B) => !A and !B
So you should use
WHERE cust_ords.[CancelDate] <> "" and cust_ords.[CancelDate] <> "//";
Or rather ISNULL(cust_ords.[CancelDate],'//')!='//'
Noow to your query itself, you are joining on dates, that is you join orders on a given date with cancellations on the same date. However you want to see orders and cancellation pr month. Since you left join cancellations you will only ever count cancellations that happen on the same date as orders!
SELECT
cust_valid.[SaleLoc]
, Format(
iif(isnull(cust_ords.[CancelDate],'//')='//'
,cust_ords.[CreationDate]
,cust_ords.[CancelDate]
,'MMMM yy') Mnth
, sum(iif(isnull(cust_ords.[CancelDate],'//'='//',1,0)) ValidOrders
, sum(iif(isnull(cust_ords.[CancelDate],'//'='//',0,1)) CancelledOrders
, sum(iif(isnull(cust_ords.[CancelDate],'//'='//',1,0))
- sum(iif(isnull(cust_ords.[CancelDate],'//'='//',0,1)) NetOrderCount
FROM
cust_ords
group by
cust_valid.[SaleLoc]
, Format(
iif(isnull(cust_ords.[CancelDate],'//')='//'
,cust_ords.[CreationDate]
,cust_ords.[CancelDate]
,'MMMM yy')
This should give you the basic data useful for pivoting, at least in SQL Server

Answer 2

THank you #Søren Kongstad for your useful explanations. I have modified / corrected the code accordingly to provide me with the needed results:
SELECT CustOrders.[Grupp namn], Format(IIf(CustOrders.[DatAnnulCde]="/ /",CustOrders.[DatCreatCde],CustOrders.[DatAnnulCde]),"yyyy-mm") AS year_month,
Sum(IIf(IsNull(CustOrders.DatCreatCde),0,1)) AS GrossOrders, Sum(IIf(CustOrders.DatAnnulCde<>"/ /",1,0)) AS CancelledOrders,
Sum(IIf(IsNull(CustOrders.DatCreatCde),0,1)) - Sum(IIf(CustOrders.DatAnnulCde<>"/ /",1,0)) AS NetOrders
FROM CustOrders
WHERE CustOrders.[CarType] = "Renault PC" And CustOrders.[DatCreatCde] >= Format(Year(Now())&"-01-01","yyyy-mm-dd")
GROUP BY CustOrders.[Grupp namn], Format(IIf(CustOrders.[DatAnnulCde]="/ /",CustOrders.[DatCreatCde],CustOrders.[DatAnnulCde]),"yyyy-mm");
The fields have different names then in the initial examples
DatAnnulCde = CancelDate
DatCreatCde = CreationDate