MS SQL SERVER LAG - sql

Question

I'm trying to apply a condition to LAG in a SQL query. Does anyone know how to do this?
This is the query:
SELECT CONCAT([FirstName],' ',[LastName]) AS employee,
CAST([ArrivalTime] AS DATE) AS date,
CAST(DATEADD(hour,2,FORMAT([ArrivalTime],'HH:mm')) AS TIME) as time,
CASE [EventType]
WHEN 20001 THEN 'ENTRY'
ELSE 'EXIT'
END AS Action,
OutTime =
CASE [EventType]
WHEN '20001'
THEN DATEDIFF(minute,Lag([ArrivalTime],1) OVER(ORDER BY [CardHolderID], [ArrivalTime]), [ArrivalTime])
ELSE
NULL
END
FROM [CCFTEvent].[dbo].[ReportEvent]
LEFT JOIN [CCFTCentral].[dbo].[Cardholder] ON [CCFTEvent].[dbo].[ReportEvent].[CardholderID] = [CCFTCentral].[dbo].[Cardholder].[FTItemID]
WHERE EventClass = 41
AND [FirstName] IS NOT NULL
AND [FirstName] LIKE 'Leeann%'
The problem I have is when the times are subtracted between two different dates, it must also be NULL when subtracting between two different dates.
The 910 is incorrect.

Answer 1

I'd add another condition to your case statement. i.e.
...
CASE
WHEN [EventType] = '20001' AND DATEDIFF(DAY,[ArrivalTime],LAG([ArrivalTime]) over (ORDER BY [CardHolderID], [ArrivalTime])) > 0
THEN NULL
WHEN [EventType] = '20001'
THEN DATEDIFF(minute,Lag([ArrivalTime],1) OVER(ORDER BY [CardHolderID], [ArrivalTime]), [ArrivalTime])
ELSE NULL

Answer 2

It seems to me that the LAG just needs to be partitioned by the date (& some other fields for good measure).
If the previous date is in another partition,
then the LAG will return NULL,
then the datediff will return NULL.
SELECT
CONCAT(holder.FirstName+' ', holder.LastName) AS employee,
CAST(repev.ArrivalTime AS DATE) AS [date],
CAST(SWITCHOFFSET(repev.ArrivalTime,'+02:00') AS TIME) as [time],
IIF(repev.EventType = 20001, 'ENTRY', 'EXIT') AS Action,
(CASE WHEN repev.EventType = 20001
THEN DATEDIFF(minute, LAG(repev.ArrivalTime)
OVER (PARTITION BY repev.EventClass, repev.CardholderID, CAST(repev.ArrivalTime AS DATE)
ORDER BY repev.ArrivalTime), repev.ArrivalTime)
END) AS OutTime
FROM [CCFTEvent].[dbo].[ReportEvent] AS repev
LEFT JOIN [CCFTCentral].[dbo].[Cardholder] AS holder ON holder.FTItemID = repev.CardholderID
WHERE repev.EventClass = 41
AND holder.FirstName LIKE 'Leeann%'
Test on db<>fiddle here