Question

Using SQL Server 2008 R2 we are looking for a way to select the shift hours that an employee has that are during the night which in the this case 22.00 and 6.00 +1.

Our problem becomes how to get the hours when the shift crosses midnight or how we get the overlap when a shift begins 05.30 to 22.30 and has an overlap in both the beginning and end of the shift.

Here is an example, theses are the data available in the database and the result we are looking for:

        startDateTime     |       endDateTime         |   nightHours
--------------------------+---------------------------+----------------
 2012-07-04 05:00:00.000    2012-07-04 23:00:00.000          2 
 2012-07-04 18:00:00.000    2012-07-05 05:00:00.000          7

Does anyone have an example or a few good pointer that we can use.

Answer 1

This may be overly complex, but it does work. We use a number of CTEs to construct useful intermediate representations:

declare @Times table (
    ID int not null,
    StartTime datetime not null,
    EndTime datetime not null
)
insert into @Times (ID,StartTime,EndTime)
select 1,'2012-07-04T05:00:00.000','2012-07-04T23:00:00.000' union all
select 2,'2012-07-04T18:00:00.000','2012-07-05T05:00:00.000'

;With Start as (
    select MIN(DATEADD(day,DATEDIFF(day,0,StartTime),0)) as StartDay from @Times
), Ends as (
    select MAX(EndTime) EndTime from @Times
), Nights as (
    select DATEADD(hour,-2,StartDay) as NightStart,DATEADD(hour,6,StartDay) as NightEnd from Start
    union all
    select DATEADD(DAY,1,NightStart),DATEADD(DAY,1,NightEnd) from Nights n
    inner join Ends e on n.NightStart < e.EndTime
), Overlaps as (
    select
        t.ID,
        CASE WHEN n.NightStart > t.StartTime THEN n.NightStart ELSE t.StartTime END as StartPeriod,
        CASE WHEN n.NightEnd < t.EndTime THEN n.NightEnd ELSE t.EndTime END as EndPeriod
    from
        @Times t
            inner join
        Nights n
            on
                t.EndTime > n.NightStart and
                t.StartTime < n.NightEnd
), Totals as (
    select ID,SUM(DATEDIFF(hour,StartPeriod,EndPeriod)) as TotalHours
    from Overlaps
    group by ID
)
select
    *
from
    @Times t
        inner join
    Totals tot
        on
            t.ID = tot.ID

Result:

ID          StartTime               EndTime                 ID          TotalHours
----------- ----------------------- ----------------------- ----------- -----------
1           2012-07-04 05:00:00.000 2012-07-04 23:00:00.000 1           2
2           2012-07-04 18:00:00.000 2012-07-05 05:00:00.000 2           7

You’ll note that I had to add an ID column in order to get my correlation to work.

The Start CTE finds the earliest applicable midnight. The End CTE finds the last time for which we need to find overlapping nights. Then, the recursive Nights CTE computes every night between those two points in time. We then join this back to the original table (in Overlaps) to find those periods in each night which apply. Finally, in Totals, we compute how many hours each overlapping period contributed.

This should work for multi-day events. You might want to change the Totals CTE to use minutes, or apply some other rounding functions, if you need to count partial hours.