TSQL - DateTime difference between more than two rows - sql-server

Question

I'm trying to find out how to calculate difference between multiple rows from one simple query. Here it is:
SELECT [DateTime],EmployeeId,ControlPointID,EventTypeID
FROM [Events]
WHERE Day([DateTime]) = 4
AND Month([DateTime]) = 7
AND Year([DateTime]) = 2017
AND EmployeeId = 451
AND ControlPointID IN ( 3, 6 )
AND EventTypeID IN ( 1, 2 )
ORDER BY [DateTime]
Result:
DateTime EmployeeId ControlPointID EventTypeID
2017-07-04 11:32:10.000 451 6 1
2017-07-04 16:07:00.000 451 3 2
2017-07-04 16:42:50.000 451 6 1
2017-07-04 20:04:10.000 451 3 2
I need to calculate difference between [DateTime] in minutes.
EventTypeId = 1 means that Employee enters to the building and EventTypeId=2 means that Employee leaves. I can calculate difference between first Enter Event and last Leave Event. In this case it's 512 minutes. But, i have problem to calculate work time, when someone enters twice and leaves twice. It should be 477 minutes. Calculation should looks like this:
DateDiff = (2017-07-04 16:07:00.000 - 2017-07-04 11:32:10.000) +
(2017-07-04 20:04:10.000 - 2017-07-04 16:42:50.000)
Can you help me figure it out, please ?

Answer 1

Given a building entry, finding the first leave after that entry can be done with cross apply:
select entry.EmployeeId, entry.DateTime, exit.DateTime
from Events entry
cross apply (select top 1 e.DateTime
from Events e
where e.EmployeeId = entry.EmployeeId
and e.DateTime > entry.DateTime
and e.EventTypeId = 2
order by e.DateTime asc
) as exit
where entry.EventTypeId = 1
at which point you just need to use the applicable T/SQL function to get the difference in whatever unit you want (eg. in minutes with datediff(minute, entry.DateTime, exit.DateTime).
To get the total of all the differences simply sum the differences:
select EmployeeId, sum(mins)
from (
select entry.EmployeeId, entry.DateTime as EntryDateTime, exit.DateTime as ExitDateTime, datediff(minute, EntryDateTime, ExitDateTime) as mins
from Events entry
cross apply (select top 1 e.DateTime
from Events e
where e.EmployeeId = entry.EmployeeId
and e.DateTime > entry.DateTime
and e.EventTypeId = 2
order by e.DateTime asc
) as exit
where entry.EventTypeId = 1
) as input
group by EmployeeId
Edit: added overall summation (with diff on the inside for clarity)

Answer 2

This can be done using LAG window function, since 2008 does not supports it we need to left join with Row_Number to find the previous entry
;WITH cte
AS (SELECT Row_number()OVER(Partition by EmployeeID ORDER BY [DateTime]) rn,*
FROM Yourresult)
SELECT a.EmployeeID,
Sum(Datediff(minute, b.[DateTime], a.[DateTime]))
FROM cte a
LEFT JOIN cte b
ON a.EmployeeID = b.EmployeeID
AND a.rn = b.rn + 1
WHERE a.[EventTypeId] = 2
GROUP BY a.EmployeeID
Note : This considers there isn't any wrong punches. Just like your sample data