Computation of period Start date - sql

Question

I have a table that hold the start date and the end date of a financial period.
CHARGE_PERIOD_ID START_DATE END_DATE
13 2013-03-31 00:00:00.000 2013-04-27 00:00:00.000
14 2013-04-28 00:00:00.000 2013-05-25 00:00:00.000
15 2013-05-26 00:00:00.000 2013-06-29 00:00:00.000
16 2013-06-30 00:00:00.000 2013-07-27 00:00:00.000
17 2013-07-28 00:00:00.000 2013-08-24 00:00:00.000
18 2013-08-25 00:00:00.000 2013-09-28 00:00:00.000
19 2013-09-29 00:00:00.000 2013-10-26 00:00:00.000
20 2013-10-27 00:00:00.000 2013-11-23 00:00:00.000
21 2013-11-24 00:00:00.000 2013-12-28 00:00:00.000
22 2013-12-29 00:00:00.000 2014-01-25 00:00:00.000
23 2014-01-26 00:00:00.000 2014-02-22 00:00:00.000
24 2014-02-23 00:00:00.000 2014-03-29 00:00:00.000
The user of a report wants the current financial year split into 12 periods and want to give to feed in 2 parameters into the report , a year and a period number which will go into my sql . So something like #year=2014 #period=1 will be recieved . I have to write some sql to go to this table and set a period start date of 31/03/2014 and a period end date of 27/04/2014.
So in pseudo code:
Look up period 1 for 2014 and return period start date of 31/03/2014 and period end date of 27/04/2014.
#PERIOD_START_DATE = select the the first period that starts in March for the given year . all financial period starts in March.
#PERIOD_END_DATE = select the corresponding END_DATE from the table .
The question is how to begin to code this or my design approach? Should I create a function that calcualtes this or should I do a CTE and add a column which will hold the period number in the way they want etc .
Thinking about it more I think I need a mapping table . So the real question is can I do this without a mapping table ?

Answer 1

DECLARE #Year INT
DECLARE #Period INT
SET #Year= 2013
SET #Period = 1
;WITH CTE AS
(
SELECT *, ROW_NUMBER() OVER (PARTITION BY
CASE WHEN MONTH([START_DATE])<3 THEN YEAR([START_DATE]) -1 ELSE YEAR([START_DATE]) END
ORDER BY
CASE WHEN MONTH([START_DATE])<3 THEN YEAR([START_DATE]) - 1 ELSE YEAR([START_DATE]) END
,CASE WHEN MONTH([START_DATE])<3 THEN MONTH([START_DATE]) + 12 ELSE MONTH([START_DATE]) END
) AS RN
FROM Periods
)
SELECT * FROM CTE
WHERE RN = #Period
AND CASE WHEN MONTH([START_DATE])<3 THEN YEAR([START_DATE]) -1 ELSE YEAR([START_DATE]) END = #Year
SQLFiddle DEMO