How to get the name and values of columns which have different values in the same table's consecutive rows - sql

Question

I have a table History with columns as follows:
HistoryId---User---Tag---Updateddate----DeptId
1 B12 abc 10-08-2017 D34
2 B24 abc 11-08-2017 D34
3 B24 def 12-08-2017 D34
I have a query
SELECT
*
FROM History
WHERE DeptId = 'D34'
ORDER BY Updateddate
The result of this query gives me the above 3 rows.
Now out of these rows I want the name and value of columns which have different values in consecutive rows.
Something like:
HistoryId-----Column-----Value
2 User B24
3 Tag def
Is there a way to do this?

Answer 1

If there are no null values for tag or user, then one way you could achieve this is by using a combination of LAG() and a CROSS APPLY to check whether any values are different.
SELECT H.HistoryID, C.Col, C.Val
FROM (
SELECT *, PrevUser = LAG([User]) OVER (ORDER BY HistoryID), PrevTag = LAG([Tag]) OVER (ORDER BY HistoryID)
FROM [History] AS H
) AS H
CROSS APPLY (
VALUES
('User', CASE WHEN PrevUser != [User] THEN [User] END),
('Tag', CASE WHEN PrevTag != Tag THEN Tag END)
) AS C(Col, Val)
WHERE C.Val IS NOT NULL;
If there are null values, it gets a bit more complicated, but the basic idea would be the same, you'd just have to add in rules to check for null values (and ignore the first row).
EDIT: If you needed to check for null values too, one way to do it would be like the following...
DECLARE #History TABLE (HistoryID INT, [User] CHAR(3), [Tag] CHAR(3));
INSERT #History VALUES
(1,'B12','abc'),
(2,'B24','abc'),
(3,'B24','def'),
(4,NULL,'def'),
(5,'A24',NULL),
(6,NULL,NULL),
(7,'123','456');
SELECT H.HistoryID, C.Col, C.Val
FROM (
SELECT *, RN = ROW_NUMBER() OVER (ORDER BY HistoryID), PrevUser = LAG([User]) OVER (ORDER BY HistoryID), PrevTag = LAG([Tag]) OVER (ORDER BY HistoryID)
FROM #History AS H
) AS H
CROSS APPLY (
VALUES
('User', CASE WHEN RN != 1 AND (PrevUser != [User] OR (PrevUser IS NULL AND [User] IS NOT NULL) OR (PrevUser IS NOT NULL AND [User] IS NULL)) THEN [User] END, CASE WHEN RN != 1 AND (PrevUser != [User] OR (PrevUser IS NULL AND [User] IS NOT NULL) OR (PrevUser IS NOT NULL AND [User] IS NULL)) THEN 1 END),
('Tag', CASE WHEN RN != 1 AND (PrevTag != Tag OR (PrevTag IS NULL AND Tag IS NOT NULL) OR (PrevTag IS NOT NULL AND Tag IS NULL)) THEN Tag END, CASE WHEN RN != 1 AND (PrevTag != Tag OR (PrevTag IS NULL AND Tag IS NOT NULL) OR (PrevTag IS NOT NULL AND Tag IS NULL)) THEN 1 END)
) AS C(Col, Val, Chk)
WHERE C.Chk = 1;

Answer 2

You can unpivot your results and then check them for change over consecutive rows like below
select
HistoryId,
column,
values
into #temp_up_values
from
(
select *
from History
where DeptId='D34'
)src
unpivot
(
values
for column
in ([User],[Tag],[Updateddate],[DeptId])
)up
select t2.*
from
#temp_up_values t1
join
#temp_up_values t2
on t1.HistoryId=t2.HistoryId +1
and t1.column=t2.column and t1.values<>t2.values