T-Sql business rule calculation - sql

Question

I have a complicated problem that needs to be solved in t-sql. Possibly without a cursor or a loop.
Given the following table with customer’s payment setup.
ID Frequency Amount Start Date End Date
1 Monthly 100 01-01-2016 N/A(ongoing)
The customer wants to know how much he\she will have to pay for the month of November.
If they were to close the account on 15 nov 2016.
For example:
Assume the customer wants to close their account on 15-nov-2016 and wants to know the $amount they will be paying from 1st of November to 15 of November.
Calculation
The frequency cycle for the customer payment is Monthly.
Taking the frequency into account we know that:
Customer start date of November will be 1st of November
The end date will be 30 November
Calculation formula
(DayUpToCloseDate/DaysInNov) * Amount = amount customer is asking.
DaysUpToCloseDate = 15 (diff 1st of nov and 15th of nov)
DaysInNov = 30
Amount = 100
(15/30)*100 = 50
So we can tell the customer he/she will be paying 50$ in November if they were to close the account on 15th of November.

Answer 1

First we need to declare the 2 variables, the date in question and the monthly amount due.
DECLARE #date datetime = '2017-11-15'
DECLARE #amountDue int = 100
Then to get the month to date amount due we can use:
SELECT CAST(DATEPART(DAY,#date) AS FLOAT)/DATEPART(DAY,DATEADD(MONTH,DATEDIFF(MONTH,0,#date)+1,0)-1) * #amountDue AS [MonthToDateAmountDue]
Here is how we got there.
SELECT
--This gets the first day of the next month and subtracts 1 day, getting the last day of the month the date falls in.
DATEADD(MONTH,DATEDIFF(MONTH,0,#date)+1,0)-1 AS [LastDayOfMonth]
--We now extract the day date part of the last day of the month to get the total days in the month.
,DATEPART(DAY,DATEADD(MONTH,DATEDIFF(MONTH,0,#date)+1,0)-1) AS [DaysInMonth]
--Finally, we get the day date part from our #date and divide by the total days of the month to get the percentage of the month we have completed.
--Since int does not do decimals, we use a float.
,CAST(DATEPART(DAY,#date) AS FLOAT)/DATEPART(DAY,DATEADD(MONTH,DATEDIFF(MONTH,0,#date)+1,0)-1) AS [PercentageOfMonthCompleted]
--And mulitply it by the amount due.
,CAST(DATEPART(DAY,#date) AS FLOAT)/DATEPART(DAY,DATEADD(MONTH,DATEDIFF(MONTH,0,#date)+1,0)-1) * #amountDue AS [MonthToDateAmountDue]
Edit: So, I just learned about the EOMONTH function. This can be shortened to
SELECT CAST(DATEPART(DAY,#date) AS FLOAT)/DATEPART(DAY,EOMONTH(#date)) * #amountDue AS AS [MonthToDateAmountDue]