TSQL Substract from report parameter to get previous month

Question

I can really use some help. I'm trying to subtract 1 from a VARCHAR report parameter that is within a CTE. I'm working in SQL 2008 R2

This works fine:

 AND MONTH(CAST(c.DATETIME AS DATE)) = CONVERT(INT,@Maand)

But this just doesn't seem to work:

AND MONTH(CAST(c.DATETIME AS DATE)) = CONVERT(INT,@Maand) -1

Thanks

Here is some of my code:

ALTER PROCEDURE [rpt].[usp_F_Telefonie_Bereikbaarheid](@Jaar AS VARCHAR(4),  @Maand AS VARCHAR(2))
AS
BEGIN

WITH CTE AS(
SELECT 
    C.DATETIME AS Datum
    ,d.NAME
    ,COUNT(Callid) AS Aantal
    ,'Vorige' AS Gesprekken 
    ,DURATIONSEC
FROM DM.dm.F_CDRS AS c
JOIN DM.dm.D_DEPARTMENTS AS d
ON d.DEPARTMENTID = c.DEPARTMENTID
WHERE Direction = 1
AND YEAR(CAST(c.DATETIME AS DATE)) = @Jaar
AND MONTH(CAST(c.DATETIME AS DATE))= CONVERT(INT,@Maand)
AND WAITTIMESEC <=10
GROUP BY LEFT(D.NAME, 2), D.NAME, C.DATETIME, DURATIONSEC
)
SELECT 
    ,CASE CTE.Gesprekken
    WHEN 'Vorige'
    THEN SUM(CTE.Aantal)
    END AS Vorige
FROM CTE
WHERE MONTH(CAST(CTE.Datum AS DATE)) = @Maand
AND YEAR(CAST(CTE.Datum AS DATE)) = @Jaar
GROUP BY CTE.NAME, CTE.Gesprekken, CTE.DURATIONSEC, CTE.Aantal

Answer 1

You need to convert varchar --> datetime and then use datediff function.

Instead this:

YEAR(CAST(c.DATETIME AS DATE)) = @Jaar
AND MONTH(CAST(c.DATETIME AS DATE))= CONVERT(INT,@Maand)

use:

datediff(month, cast(@Jaar + right('0' + @Maand, 2) + '01' as datetime), c.[datetime]) = 1