Breaking out yearly payments into monthly payments with month name in a 3 year period - sql

Question

I was wondering where to go from my initial idea. I used the query below to get the month beginning dates for each of the three years:
DECLARE #STARTDATE DATETIME,
#ENDDATE DATETIME;
SELECT #STARTDATE='2013-01-01 00:00:00.000',
#ENDDATE='2015-12-31 00:00:00.000';
WITH [3YearDateMonth]
AS
(
SELECT TOP (DATEDIFF(mm,#STARTDATE,#ENDDATE) + 1)
MonthDate = (DATEADD(mm,DATEDIFF(mm,0,#STARTDATE) + (ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) -1),0))
FROM sys.all_columns ac1
)
SELECT MonthDate
FROM [3YearDateMonth]
I am not sure if I should DATENAME(Month, Monthdate) it later for the month names or just do it in the cte; any suggestions would be great.
My data looks like this:
BeginDate EndDate Payment
2013-01-01 00:00:00.000 2013-12-31 00:00:00.000 3207.70
2014-01-01 00:00:00.000 2014-12-31 00:00:00.000 3303.93
2015-01-01 00:00:00.000 2015-12-31 00:00:00.000 3403.05
Since the payment is yearly I can use payment/12 to get an average monthly amount. I want my data to look like this:
BeginDate EndDate Month MonthlyAmount
2013-01-01 00:00:00.000 2013-01-31 00:00:00.000 January 267.3083
2013-02-01 00:00:00.000 2013-02-31 00:00:00.000 February 267.3083
...
2014-01-01 00:00:00.000 2014-01-31 00:00:00.000 January 275.3275
2014-02-01 00:00:00.000 2014-02-31 00:00:00.000 February 275.3275
...
2015-01-01 00:00:00.000 2015-01-31 00:00:00.000 January 283.5875
2015-02-01 00:00:00.000 2015-02-31 00:00:00.000 February 283.5875
All the way through December for each yearly pay period.
I will be pivoting the Month column later to put the monthly amounts under the corresponding month they belong in.
Is this doable because I feel lost at this point?

Answer 1

Starting with your three data rows, you can use the following query to get your desired results:
with months as
(
select BeginDate
, EndDate
, Payment = Payment / 12.0
from MyTable
union all
select BeginDate = dateadd(mm, 1, BeginDate)
, EndDate
, Payment
from months
where dateadd(mm, 1, BeginDate) < EndDate
)
select BeginDate
, EndDate = dateadd(dd, -1, dateadd(mm, 1, BeginDate))
, Month = datename(mm, BeginDate)
, MonthlyAmount = Payment
from months
order by BeginDate
SQL Fiddle with demo.

Answer 2

Here's a query for you:
WITH L1 (N) AS (SELECT 1 UNION ALL SELECT 1),
L2 (N) AS (SELECT 1 FROM L1, L1 B),
L3 (N) AS (SELECT 1 FROM L2, L2 B),
Num (N) AS (SELECT Row_Number() OVER (ORDER BY (SELECT 1)) FROM L3)
SELECT
P.BeginDate,
P.EndDate,
M.MonthlyPayDate,
MonthlyAmount =
CASE
WHEN N.N = C.MonthCount
THEN P.Payment - Round(P.Payment / C.MonthCount, 2) * (C.MonthCount - 1)
ELSE Round(P.Payment / C.MonthCount, 2)
END
FROM
dbo.Payment P
CROSS APPLY (
SELECT DateDiff(month, BeginDate, EndDate) + 1
) C (MonthCount)
INNER JOIN Num N
ON C.MonthCount >= N.N
CROSS APPLY (
SELECT DateAdd(month, N.N - 1, BeginDate)
) M (MonthlyPayDate)
ORDER BY
P.BeginDate,
M.MonthlyPayDate
;
See a Live Demo at SQL Fiddle
Pluses:
Doesn't assume 12 months--it will work with any date range.
Properly rounds all non-ultimate months, then assigns the remainder to the last month so that the total sum is accurate. For example, for 2013, the normal monthly payment is 267.31, but December's month's payment is 267.29.
Minuses:
Assumes all dates entirely enclose full months, starting on the 1st and ending on the last day of the month.
If you provide more detail about further requirements regarding pro-rating, I can improve the query for you.