Question

I’ve got a query and I want to add parameters to calculate the moving annual (the figures for the last 12 months). I’m trying to subtract 12 months from today, so if for instance today is 1 August 2012 then my @StartDate should be ‘2011-09-01’ and my @EndDate should be ‘2012-08-31’. So how do I change set my parameters to accommodate this?

declare @StartDate DATE
declare @EndDate   DATE
SET @StartDate = DATEADD(MONTH, -12, '2012-08-01')
SET @EndDate = DATEADD(MONTH, +1, '2012-08-01')

Answer 1

DECLARE
  @InputDate DATE,
  @StartDate DATE,
  @EndDate   DATE

SET
  @InputDate = '2012-08-01'

SET
  @StartDate = DATEADD(MONTH, DATEDIFF(MONTH, 0, @InputDate) - 11, 0)

SET
  @EndDate   = DATEADD(DAY, -1, DATEADD(MONTH, 12, @StartDate))

EDIT:

However, I do not recomend using '2011-09-01' to '2012-08-31' as a representation of a year. Instead, use '2011-09-01' to '2012-09-01' in the following way…

WHERE
      table.dateField >= '2011-09-01'
  AND table.dateField <  '2012-09-01'

This works for all Date and DateTime data types. Even if the value in dateField is 2:30pm on 31st August, this will still work. It’s a one size fits all approach and makes it much more difficult to make mistakes on date and time boundaries.