Question

I have a table with rows like this:

   ID  StatusId   Date
    1    1        2001-01-01
    2    1        2001-01-02
    3    2        2001-01-03
    4    3        2001-01-04
    5    1        2001-01-05
    6    2        2001-01-06
    7    2        2001-01-07
    8    1        2001-01-08
    9    1        2001-01-09

I need to get the date when the current value of the status was originally changed. For the above example, the last value is 1, and it’s changed in row 8, so the result would be 2001-01-08.

How would you do this?

If you need a table to test with, here it is:

DECLARE @Tbl AS TABLE (ID INT, StatusId INT, Date DATETIME)

INSERT INTO @Tbl(ID, StatusId, Date)
SELECT 1,1,'2001-01-01' UNION
SELECT 2,1,'2001-01-02' UNION
SELECT 3,2,'2001-01-03' UNION
SELECT 4,3,'2001-01-04' UNION
SELECT 5,1,'2001-01-05' UNION
SELECT 6,2,'2001-01-06' UNION
SELECT 7,2,'2001-01-07' UNION
SELECT 8,1,'2001-01-08' UNION
SELECT 9,1,'2001-01-09' 

    SELECT * FROM @Tbl

Answer 1

This one should get you what you’re after:

declare @LastStatusID int
declare @LastDate datetime
declare @LastID int

declare @LastChangeID int

/* get last record */
select top 1 @LastStatusID = StatusID, @LastDate = Date, LastID = ID
from @Tbl
order by ID desc

/* get last record with a different status */
select top 1 @LastChangeID = ID
from @Tbl
where ID < @LastID and StatusID <> @LastStatusID
order by ID desc

/* get the first next record - this would get you the last record as well whe it's just been set */
select top 1 Date
from @Tbl
where ID > @LastChangeID
order by ID asc

I haven’t included any checking for margin examples when there’d be just one record in the table or multiple of them but all with the same status. You can figure those out yourself.

As a single query

This query requires IDs without gaps and it will get you the last record after a status change and it will also work when there’s just one record in the table or multiple of them with the same status (isnull provides the required functionality)

select top 1 Date
from @tbl t1
    left join @tbl t2
    on (t2.ID = t1.ID - 1)
where (isnull(t2.StatusID, -1) <> t1.StatusID)
order by ID desc

Last where clause changes a null value (when there’s no upper record) to -1. If you do have a status with this value, you should change this number to some non-existing status value.