Getting each status change in a table

Question

I have a table that holds status changes per item. The item can go in and out of a status multiple times.

ItemTable (ItemId int, Status varchar(50), DateOfChange date)

I need to pull the date of each change. However, since the statuses can be repeated for each Item, I can't just do the min(DateOfChange) and find the occurrences.

This is in an OLAP environment, and the subquery that's been used to pull this data is KILLING performance.

Is there a way to pull this stuff via partitioning/rownumber functions? Or something else that would behave better in OLAP? (This is in SQL 2008.)

Answer 1

This type of requirement comes under the banner of "gaps and islands". A popular approach is

WITH T
     AS (SELECT *,
                DENSE_RANK() OVER (PARTITION BY ItemId ORDER BY DateOfChange) - 
                DENSE_RANK() OVER (PARTITION BY ItemId, Status ORDER BY DateOfChange) AS Grp
         FROM   ItemTable)
SELECT ItemId,
       Status,
       MIN(DateOfChange) AS Start,
       MAX(DateOfChange) AS Finish
FROM   T
GROUP  BY ItemId,
          Status,
          Grp
ORDER  BY Start