Pivot n times for Select statement in T-SQL - sql

Question

DECLARE
#StartDate DATE,
#EndDate DATE,
#NumMonths INT,
SET #NumMonths = 12
SET #StartDate = DATEADD(m, -#NumMonths, DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()), 0))
SET #EndDate = DATEADD(m, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()), 0))
SELECT ROW_NUMBER() OVER (ORDER BY SignUpDate) RowNum, SignUpDate,
TotalSignUp / TotalSignUp M1,
--M2 / TotalSignUp M2,
--M3 / TotalSignUp M3,
--M4 / TotalSignUp M4,
--M5 / TotalSignUp M5,
--M6 / TotalSignUp M6,
--M7 / TotalSignUp M7,
--M8 / TotalSignUp M8,
--M9 / TotalSignUp M9,
--M10 / TotalSignUp M10,
--M11 / TotalSignUp M11,
--M12 / TotalSignUp M12
FROM (
SELECT SignUpDate,
COUNT(DISTINCT ClientID) TotalSignUp,
SUM(CASE WHEN MonthActive = DATEADD(m, 1, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M2,
SUM(CASE WHEN MonthActive = DATEADD(m, 2, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M3,
SUM(CASE WHEN MonthActive = DATEADD(m, 3, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M4,
SUM(CASE WHEN MonthActive = DATEADD(m, 4, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M5,
SUM(CASE WHEN MonthActive = DATEADD(m, 5, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M6,
SUM(CASE WHEN MonthActive = DATEADD(m, 6, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M7,
SUM(CASE WHEN MonthActive = DATEADD(m, 7, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M8,
SUM(CASE WHEN MonthActive = DATEADD(m, 8, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M9,
SUM(CASE WHEN MonthActive = DATEADD(m, 9, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M10,
SUM(CASE WHEN MonthActive = DATEADD(m, 10, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M11,
SUM(CASE WHEN MonthActive = DATEADD(m, 11, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0)) THEN 1.0 ELSE 0.0 END) M12
How can I perform a loop for the select statement so that I can dynamically produce the number of M select statements as set by the #numMonths variable?
e.g., I will want the user of the report to be able to select the #numMonths as an input variable and they could say, want to go back 6 months, or 24 months. I would want the select statement to dynamically pivot in a loop depending on what the #numMonths value is.
To put it in other words I don't want to type out M1-M36 36 times just in case the user wants to go back 3 years, that will look so messy. I want a 1 line 'template' and a Counter + 1 until it reaches the number of #numMonths

Answer 1

DROP TABLE IF EXISTS ##TimePeriods
GO
DECLARE
#StartDate DATE,
#EndDate DATE,
#NumMonths INT,
#SQL NVARCHAR(MAX)
SET #NumMonths = 5
SET #StartDate = DATEADD(m, -#NumMonths, DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()), 0))
SET #EndDate = DATEADD(m, -1, DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()), 0))
;WITH TimePeriods AS
(
SELECT #StartDate AS TheDate
UNION ALL
SELECT DATEADD(m,1,TheDate) FROM TimePeriods
WHERE TheDate < #EndDate
)
SELECT * INTO ##TimePeriods FROM TimePeriods
DECLARE #ColList VARCHAR(MAX) = ''
SELECT #ColList = STRING_AGG(QUOTENAME(TheDate),',')
FROM ##TimePeriods
SET #SQL =
'
SELECT
*
FROM
(
SELECT 1 AS TheData, *
FROM ##TimePeriods
-- JOIN YOUR DATA IN HERE
) SourceData
PIVOT
(MAX(TheData)
FOR TheDate IN ('+#ColList+')
)
AS PivotTable
'
EXEC (#SQL)