SQL Creating a function which checks variables to make a total - sql

Question

I've made 2 views which calculates the profit per reservation.
One calculates the profit from bikes and the other one from accessories.
Now when I tried SUMing them together, I get false reports. I found out why: something a reservation doesnt contain a bike or a accessory. This field gives a NULL return, which makes my SUM function unusable, because it lacks results.
I tried this:
USE Biker
GO
CREATE FUNCTION fnMaxOmzet
(
#Fiets AS int,
#Accessoire AS int
)
RETURNS int
AS
BEGIN
DECLARE #MaxOmzet AS int
IF #Fiets = NULL
SET #MaxOmzet = #Accessoire
ELSE IF #Accessoire = NULL
SET #MaxOmzet = #Fiets
ELSE
SET #MaxOmzet = #Fiets + #Accessoire
RETURN #MaxOmzet
END
But it isn't working because it gives multiple results..
Probably cause I am using '=' while it checking a list.
Does anyone know a way to make this function?
SELECT * FROM dbo.vAccessoireOmzet
SELECT * FROM dbo.vFietsOmzet
https://ibb.co/bTmJ26
Expected result: List of ID 1 - 100 and the total of AccesoireOmzet + FietsOmzet

Answer 1

Personally, I would use an inline Table Function. You'll find the performance is significantly faster.
NExt, you don't test for the value NULL by using the = operator, you use IS NULL.
This should cater for the logic you have above, however it is untested due to missing DDL and sample data:
CREATE FUNCTION dbo.fnMaxOmzet (#Fiets int, #Accessoire int)
RETURNS TABLE AS
RETURN
(
SELECT CASE WHEN #Fiets IS NULL THEN #Accessoire
WHEN #Accessoire IS NULL THEN #Fiets
ELSE #Fiets + #Accessoire
END AS MaxOmzet
)
GO
OK, just seen your comment with this sql: SELECT dbo.fnMaxOmzet((SELECT FietsOmzet FROM dbo.vFietsOmzet), (SELECT AccessoireOmzet FROM dbo.vAccessoireOmzet)).
That syntax is just wrong, sorry. You need to use APPLY and a JOIN. This is pure guess work as we have no DDL and Sample data... however:
SELECT MO.MaxOmzet--, --**Your other columns go here**
FROM vFietsOmzet FO
JOIN vAccessoireOmzet AO ON --**Your JOIN criteria goes here**
CROSS APPLY dbo.fnMaxOmzet (FO.FietsOmzet, AO.AccessoireOmzet) MO;

Answer 2

You need to use IS NULL rather than = NULL.
But you could also use something like this:
USE Biker
GO
CREATE FUNCTION fnMaxOmzet
(
#Fiets AS int,
#Accessoire AS int
)
RETURNS int
AS
BEGIN
RETURN ISNULL(#Fiets, 0) + ISNULL(#Accessoire, 0)
END
You can read here for more information on the behaviour of = NULL.
Update:
I imagine what you might be looking for is something like:
SELECT
ISNULL(F.Huurovereenkomst_id, A.Huurovereenkomst_id) Huurovereenkomst_id
, SUM(dbo.fnMaxOmzet(F.FietsOmzet, A.AccessoireOmzet)) MaxOmzet
FROM
dbo.vFietsOmzet F
FULL JOIN dbo.vAccessoireOmzet A ON F.Huurovereenkomst_id = A.Huurovereenkomst_id
GROUP BY ISNULL(F.Huurovereenkomst_id, A.Huurovereenkomst_id)

Answer 3

use coalesce thats position
USE Biker
GO
CREATE FUNCTION fnMaxOmzet
(
#Fiets AS int,
#Accessoire AS int
)
RETURNS int
AS
BEGIN
RETURN coalesce(#Fiets+#Accessoire,#Fiets,#Accessoire) from MyTableVar;
END