Question

I would like to create a bar chart displaying the number of objects that were a available on a monthly base. All rows have a start and end date. I know how to do the count for a single month:

SELECT COUNT(*) As NumberOfItems
FROM Items
WHERE DATEPART(MONTH, Items.StartDate) <= @monthNumber 
AND DATEPART(MONTH, Items.EndDate) >= @monthNumber

Now I would like do create the SQL to get the month number and the number of items using a single SELECT statement.

Is there any elegant way of accomplishing this? I am aware I have to take the year number into account.

Answer 1

Assuming Sql Server 2005 or newer.

CTE part will return month numbers spanning years between @startDate and @endDate. Main body joins month numbers with items performing the same conversion on Items.StartDate and Items.EndDate.

; with months (month) as (
  select datediff (m, 0, @startDate)
  union all
  select month + 1
    from months
  where month < datediff (m, 0, @endDate)
)
select year (Items.StartDate) Year,
       month (Items.StartDate) Month,
       count (*) NumberOfItems
  from months
  inner join Items
     on datediff (m, 0, Items.StartDate) <= months.month
    and datediff (m, 0, Items.EndDate) >= months.month
  group by 
       year (Items.StartDate),
       month (Items.StartDate)

Note: if you intend to span more than hundred months you will need option (maxrecursion 0) at the end of query.