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.)

  • To clarify, you need to pull the first date of each status for an item? – Kevin Jan 11 '14 at 14:01
  • Sorry, power knocked out the office for about 10 hours. :P No, I need each date for each status change. So if it goes into status A 5 times, I need 5 dates. Be nice if it were just the first instance of A, but no such luck. – Valkyrie Jan 12 '14 at 1:21
  • I had an answer lined up but Martin's is fleshed out a little better if that's what you were looking for. What doesn't quite make sense to me is you have a date per status per item in your table, what are you looking for? – Kevin Jan 12 '14 at 1:28
  • 'Cause it's not just those columns; it's a metric ton of data per day, but this one column is all I care about for this crappy subquery. Martin saved my bacon! – Valkyrie Jan 12 '14 at 2:28

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

     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)
       MIN(DateOfChange) AS Start,
       MAX(DateOfChange) AS Finish
ORDER  BY Start 
| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.