user-defined scalar function to generate computed column - sql-server

Question

I have databse having 2 tables i.e (Installment and InstallmentPlan).
In the installment table there are number of columns. I want to add one new computed column name SurchargeCalculated (SC). The calculation for SC is this
SC = (Installment.Amount * Installment.days * (InstallmentPlan.InstPercentage /365 /100))
I have created user-defined function SurchargeCal having 3 parameters which are Amount, days and InstPercentage. The problem is that when i add computed column in installment table and call scalar function from there, the saclar func needs InstPercetage parameter of 2nd table (InstallmentPlan).
I know recommended ways is to use view but that will complicate my problem as i am using installment table in C#.
Any help will be extremely appreciated.
My scalar function is
USE [myDB]
GO
/****** Object: UserDefinedFunction [dbo].[SurchargeCal] Script Date: 17/02/2020 2:21:15 PM
******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
ALTER FUNCTION [dbo].[SurchargeCal]
(
-- Add the parameters for the function here
#days as int,
#amount as money,
#surchargePerc as decimal
)
RETURNS decimal
AS
BEGIN
-- Declare the return variable here
DECLARE #result as decimal =0;
-- Add the T-SQL statements to compute the return value here
--SELECT <#ResultVar, sysname, #Result> = <#Param1, sysname, #p1>
if #days = 0
set #result = 0
else if (#days > 0 and #amount > 0)
set #result = (#days * #amount * (#surchargePerc/ 365 / 100))
else
set #result = 0
-- Return the result of the function
RETURN #result
END
then below ADD table command XXXX is the problem
USE [myDB]
GO
ALTER TABLE dbo.Installment
ADD SurchargeCalculated AS dbo.SurchargeCalc(days,Amount, XXXX) --where XXX should be InstPercenatage
GO

Answer 1

Since you can't use a subquery directly on the computed column, you will need to do it on the scalar function itself :
CREATE FUNCTION SurchargeCal(#days as integer, #amount as money, #PlanKey as integer)
RETURNS decimal
AS
BEGIN
DECLARE #result as decimal = 0;
SELECT #result = #amount * #days * InstPercentage / 365 / 100
FROM InstallMentPlan
WHERE PlanKey = #PlanKey
RETURN #result
END
Now you can create the computed column, passing the PlanKey instead of its InstPercentage.
ALTER TABLE dbo.Installment
ADD SurchargeCalculated AS dbo.SurchargeCalc(days, Amount, PlanKey)