MSSQL: Select max date when a specific column was updated - sql-server

Question

I need to return the last records which had a certain column updated.
The data is stored in 2 tables: ClientInfo and ClientInfoHistory both of them with a large amount of data, especially the history one (which keeps track on any changes on the main table ClientInfo). Even if only one column was updated in ClientInfo, a row with the version before that update will be inserted into ClientInfoHistory with audit info (the user and the date when that row was updated).
So I am interested in returning the max date when the Flag3 was changed for each ClientID + the user who changed that and the value of Flag3
The entire script should be in a stored proc avoiding perf issues.
My idea:
I create a temp table with the entire customer's history + actual data, then I compare the Flag3 to see when that flag was changed for each customer and throw it into another temp table along with the modifiedbyuserid and modifieddate. Then I select max(modifiedDate) for each customer.
The tables look like this:
create table dbo.ClientInfo(
ClientID int,
Flag1 bit,
Flag2 bit,
Flag3 bit,
CreatedByUserID int not null,
CreatedDate datetime not null default getdate(),
ModifiedByUserID int not null,
ModifiedDate datetime not null default getdate(),
constraint PK_UserInfo primary key (ClientID)
)
create table dbo.ClientInfoHistory(
ClientInfoHistoryID bigint identity not null,
ClientID int,
Flag1 bit,
Flag2 bit,
Flag3 bit,
CreatedByUserID int not null,
CreatedDate datetime not null,
HistoryStartModifiedByUserID int not null,
HistoryStartDate datetime not null,
HistoryEndModifiedByUserID int not null,
HistoryEndDate datetime not null,
constraint PK_ClientInfoHistory primary key (ClientInfoHistoryID)
)
CREATE NONCLUSTERED INDEX ix_ClientInfoHistory_ClientID_HistoryDates
on dbo.ClientInfoHistory (ClientID, HistoryStartDate, HistoryEndDate)
The temp table with the entire customer's history + actual data contains this:
SELECT c.ClientID
,c.Flag3
,c.ModifiedDate as HistoryStartDate
,'9999-12-31' as HistoryEndDate
,c.ModifiedByUserId as HistoryStartModifiedByUserID
FROM dbo.ClientInfo c with (nolock)
UNION
SELECT
ch.ClientID
,ch.Flag3
,ch.HistoryStartDate
,ch.HistoryEndDate
,ch.HistoryStartModifiedByUserID
FROM dbo.ClientInfoHistory ch with (nolock)
insert into dbo.ClientInfo (ClientID, Flag1, Flag2, Flag3, CreatedByUserID, CreatedDate, ModifiedByUserID, ModifiedDate)
values
(1,0,0,1,1234,'2019-05-29 04:57:13.360', 1111,'2019-07-01 08:35:13.360'),
(2,0,1,0,1233,'2019-03-12 04:57:13.360', 1233, '2019-03-12 04:57:13.360')
insert into dbo.ClientInfoHistory (ClientID, Flag1, Flag2, Flag3, CreatedByUserID, CreatedDate, HistoryStartModifiedByUserID,HistoryStartDate, HistoryEndModifiedByUserID, HistoryEndDate )
values
(1,1,1,1,1234,'2019-05-29 04:57:13.360', 1234, '2019-05-29 04:57:13.360', 4321,'2019-05-31 04:57:13.360'),
(1,0,1,1,1234,'2019-05-29 04:57:13.360', 4321,'2019-05-31 04:57:13.360', 9871,'2019-06-02 06:27:13.360'),
(1,0,1,0,1234,'2019-05-29 04:57:13.360',9871,'2019-06-02 06:27:13.360', 8765,'2019-06-21 11:06:13.360'),
(1,0,0,0,1234,'2019-05-29 04:57:13.360',8765,'2019-06-21 11:06:13.360', 9871,'2019-06-22 06:27:13.360')
(1,0,0,1,1234,'2019-05-29 04:57:13.360',9871,'2019-06-22 06:27:13.360', 4321,'2019-06-25 08:35:13.360'),
(1,0,1,1,1234,'2019-05-29 04:57:13.360',4321,'2019-06-25 08:35:13.360', 1111,'2019-07-01 08:35:13.360')
So for example above...The changes of that flag are:
ClientID,Flag3,ModifiedByUserID,ModifiedDate
1, 1, 1234, 29/05/2019
1, 0, 9871, 02/06/2019
1, 1, 9871, 22/06/2019***
2, 0, 1233, 12/03/2019***
and the final set of results should return only those rows marked with *** (the last version of Flag3 for each ClienID)
I am not so happy with the enitre process I chose and I am looking for another solution with a better performance. I would be really happy to hear some ideas from you guys

Answer 1

Following query must give you expected result
SELECT *
FROM (
SELECT *, ROW_NUMBER () OVER (PARTITION BY ch.ClientID ORDER BY ch.HistoryStartDate DESC) as Rn
FROM
(
SELECT c.ClientID
,c.Flag3
,c.ModifiedDate as HistoryStartDate
,'9999-12-31' as HistoryEndDate
,c.ModifiedByUserId as HistoryStartModifiedByUserID
-- ,ROW_NUMBER () OVER (PARTITION BY c.ClientID ORDER BY c.ModifiedDate DESC) as Rn
FROM dbo.ClientInfo c with (nolock)
UNION
SELECT
ch.ClientID
,ch.Flag3
,ch.HistoryStartDate
,ch.HistoryEndDate
,ch.HistoryStartModifiedByUserID
--,ROW_NUMBER () OVER (PARTITION BY ch.ClientID ORDER BY ch.HistoryStartDate DESC) as Rn
FROM dbo.ClientInfoHistory ch with (nolock)
) as CH
) AS T
WHERE T.Rn = 1

Answer 2

Your question was not exactly clear, but I believe this is what you are looking for.
You will probably want to change your indexing around a little to include Flag3, and depending on the size of your data you might consider indexing the temp table. But this is a quick and dirty query to get you started
;WITH CTE AS
(select t2.ClientID, MAX(HistoryStartDate) NotMatchDate
from #ClientInfo t1
INNER JOIN #ClientInfoHistory t2 on t1.ClientID = t2.ClientID and t1.Flag3 <> t2.Flag3
GROUP BY t2.ClientID
)
select t2.ClientID, MIN(HistoryStartDate) ModifiedDate
INTO #tmpClient
from #ClientInfo t1
INNER JOIN #ClientInfoHistory t2 on t1.ClientID = t2.ClientID and t1.Flag3 = t2.Flag3
INNER JOIN CTE t3 on t3.ClientID = t1.ClientID
WHERE t2.HistoryStartDate > t3.NotMatchDate
GROUP BY t2.ClientID
SELECT t1.ClientID, t1.Flag3,t1.HistoryStartModifiedByUserID ModifiedByUserID, t1.HistoryStartDate ModifiedDate
FROM #ClientInfoHistory t1
INNER JOIN #tmpClient t2 on t1.ClientID = t2.ClientID and t1.HistoryStartDate = t2.ModifiedDate
UNION ALL
SELECT t1.ClientID, Flag3,CreatedByUserID ModifiedByUserID,CreatedDate ModifiedDate
FROM #ClientInfo t1
LEFT JOIN #tmpClient t2 on t1.ClientID = t2.ClientID
WHERE t2.ClientID IS NULL
EDIT #2: Fixed for new rows