Question

The problem is that it takes way to long in SQL and there must be a better way. I’ve picked out the slow part for the scenario bellow.
Scenario:
Two (temp) tables with event times for start and end for vehicles that have to be paired up to figure idle durations. The issue is that some of the event data is missing. I figured out a rudimentary way of going through and determining when the last end time is after the next start time and removing the invalid start. Again not elegant + very slow.

Tables :

create table #start(VehicleIp int null,                                         CurrentDate datetime null,
                    EventId int null, 
                    StartId int null)
create table #end(VehicleIp int null, 
        CurrentDate datetime null,
        EventId int null, 
        EndId int null)

–//Note: StartId and EndId are both pre-filled with something like:

ROW_NUMBER() Over(Partition by VehicleIp order by VehicleIp,  CurrentDate)

–//Slow SQL

while exists(
            select top 1 tOn.EventId 
            from #start as tOn 
            left JOIN #end tOff 
            on tOn.VehicleIp = tOff.VehicleIp and 
            tOn.StartID = tOff.EndID +1
     )
begin

      declare @badEntry int

      select top 1 @badEntry = tOn.EventId  
      from #s as tOn 
            left JOIN #se tOff  
              on tOn.VehicleIp = tOff.VehicleIp and 
                 tOn.StartID = tOff.EndID +1 
       order by tOn.CurrentDate 

       delete from #s where EventId  = @badEntry 

      ;with _s as ( select VehicleIp, CurrentDate, EventId, 
                           ROW_NUMBER() Over(Partition by VehicleIp 
                           order by VehicleIp, CurrentDate) StartID 
                     from #start)
      update #start 
  set StartId = _s.StartId 
  from #s join _s on #s.EventId  = _s.EventId 

end 

Answer 1

Assuming you start with a table containing Vehicle and interval in which it was used, this query will identify gaps.

select b.VehicleID, b.IdleStart, b.IdleEnd
from
   (
      select VehicleID, 
       -- If EndDate is not inclusive, remove +1
             EndDate + 1 IdleStart, 
       -- First date after current for this vehicle
       -- If you don't want to show unused vehicles to current date remove isnull part 
             isnull((select top 1 StartDate 
                from TableA a 
               where a.VehicleID = b.VehicleID
                 and a.StartDate > b.StartDate
               order by StartDate
             ), getdate()) IdleEnd
        from TableA b
   ) b
where b.IdleStart < b.IdleEnd

If dates have time portion they should be truncated to required precision, here is for day:

dateadd(dd, datediff(dd,0, getDate()), 0)

Replace dd with hh, mm or whatever precision is needed.

And here is Sql Fiddle with test