Question

I have first table with defined work times: FreeTimes table (start datetime, end datetime) and second table with already planned tasks: TaskTime table (start datetime, end datetime).

I need somehow to subtract second table from first, so i get a result set of remainig FreeTimes under this conditions:

• in case where TaskTime (or more TaskTimes) is in middle of FreeTime, i need to split FreeTime to time before task and time ramaining after task
• in case when TaskTime is overlapping entire FreeTime i need to filter out this FreeTime
• in case where TaskTime is intersecting with FreeTime i need to subtract TaskTime from FreeTime and leave only remaing part of FreeTime

Answer 1

The general way I’d solve this is as follows:

;With AllTimes as (
    select [Start] as EventTime from FreeTimes
    union
    select [End] from FreeTimes
    union
    select [Start] from TaskTimes
    union
    select [End] from TaskTimes
), OrderedTimes as (
    select EventTime,ROW_NUMBER() OVER (ORDER BY EventTime) rn
    from AllTimes
), Intervals as (
    select
        ot1.EventTime as StartTime,
        ot2.EventTime as EndTime
    from
        OrderedTimes ot1
            inner join
        OrderedTimes ot2
            on
                ot1.rn = ot2.rn - 1
)
select * from Intervals i
where not exists (
    select * from TaskTimes T where --Overlapped
       T.[Start] < i.EndTime and
       T.[End] > i.StartTime)
and exists (
    select * from FreeTimes T where
       T.[Start] < i.EndTime and
       T.[End] > i.StartTime)

Where we basically order all datetime values of interest, and then for each pair of successive values, work out if there’s some overlap with the TaskTimes table. If there is, then that pair shouldn’t be in the final result. (edit – we also have to check that the interval pair does actually overlap with FreeTimes too)

You can, if needed, take this further and merge intervals (if there are overlapping rows in FreeTimes, you may end up with multiple intervals which are adjacent to each other)