Creating a SQL query to find recent field changes - sql

Question

I am having problems trying to create a SQL query to select the most recent change in hours and the difference from the previous time recorded.
The table is as below, the database keeps all the historical version together by versions:
Item ID Title RevisedDate ChangedDate Rev WorkHours
Task 187061 Development 10/9/12 11:14 10/5/12 15:54 1 4
Task 187061 Development 10/9/12 14:29 10/9/12 11:14 2 8
Task 187061 Development 10/10/12 15:07 10/9/12 14:29 3 16
Task 187061 Development 10/11/12 9:59 10/10/12 15:07 4 16
Task 187061 Development 10/12/12 10:51 10/11/12 9:59 5 16
Task 187061 Development 12/6/12 15:25 10/12/12 10:51 6 16
Task 187061 Development 12/11/12 10:27 12/6/12 15:25 7 16
Task 187061 Development 1/1/99 0:00 12/11/12 10:27 8 16
So the task most recent worked hours were updated on 10/10/12 15:07 from 8hr to 16hrs. I am having problems creating a query to tell me.
At the end of the day I need a result :-
Item ID Title RevisedDate ChangedDate Rev WorkHours ChangeHours
Task 187061 Development 10/10/12 15:07 10/9/12 14:29 3 16 8
(p.s I took one task as an example, the actual table has hundreds of task and several historical version)

Answer 1

As I understand your question you want the item with the most recent revised date for each ID
You get that like this:
SELECT *
FROM
(
SELECT *,
ROW_NUMBER() OVER (PARTITION BY ID ORDER BY RevisedDate DESC) as ord
FROM TABLE
) T
WHERE ord = 1
If you want the first one that changed that is harder:
-- First find the ones that changed
With FlagChange AS
(
SELECT T1.ID, T1.REV, T1.RevisedDate
CASE WHEN T2 IS NULL THEN FALSE
WHEN T1.WorkHour != T2.WorkHour THEN TRUE
ELSE FALSE END AS Changed
FROM TABLE T1
LEFT JOIN TABLE T2 ON T1.ID = T2.ID AND T2.REV = T1.REV-1
), NumberChange -- now use row number
(
SELECT ID, REV,
ROW_NUMBER() OVER (PARTITION BY ID ORDER BY RevisedDate DESC) as ord
FROM FlagChange
WHERE Changed = True
), SelectRecent -- take the newest ones
(
SELECT ID, REV
FROM NumberChange
WHERE ord = 1
) -- add in all the data and the ones with one revision
SELECT *
FROM TABLE T1
JOIN SelectRecient SR ON T1.ID = SR.ID AND T1.REV = SR.REV
UNION ALL
SELECT *
FROM TABLE
WHERE ID NOT IN (SELECT ID FROM SelectRecent)