Question

I’ve got a table that holds the location and timestamp information from a GPS module. The table looks something like this:

declare @VehicleData table
(
    [TimeStamp] DateTime,
    Latitude float,
    Longitude float
)

insert into @VehicleData
values  ('2012/03/06 10:00', 1, 1),
        ('2012/03/06 10:01', 1, 2),
        ('2012/03/06 11:00', 0, 0),
        ('2012/03/06 11:01', 0, 0),
        ('2012/03/06 11:02', 2, 2),
        ('2012/03/06 11:03', 2, 2),
        ('2012/03/06 11:04', 2, 3),
        ('2012/03/06 11:20', 0, 0),
        ('2012/03/06 11:21', 5, 5),
        ('2012/03/06 11:22', 5, 6),
        ('2012/03/06 11:23', 5, 6),
        ('2012/03/06 11:24', 5, 7),
        ('2012/03/06 11:25', 5, 8)

You’ll notice that some of the data points have latitude and longitude values of 0, due to the GPS module not have a fix at the time. What I’d like to do when retrieving data points from this table is to set all zero latitude and longitude values to the last non-zero latitude and longitude values. I currently do this in my client side code, but I’d like to integrate this into my database stored procedures. The client side code looks something like this:

Dim StartDate = '2012/03/06 11:00'
Dim EndDate = '2012/03/06 11:25'

Dim Results = GetVehicleDataBetweenDates(StartDate, EndDate)

Dim LastKnownLatitude = 0, LastKnownLongitude = 0

If Results(0).Latitude = 0 Then
    'Query database for last non-zero position before StartDate
    GetLastKnownPositionBeforeDate(StartDate, LastKnownLatitude , LastKnownLongitude )
Else
    LastKnownLatitude = Results(0).Latitude
    LastKnownLongitude = Results(0).Longitude
EndIf

For i = 0 to Results.Count - 1
    If Results(i).Latitude = 0 Then
        Results(i).Latitude = LastKnownLatitude
        Results(i).Longitude = LastKnownLongitude
    Else
        LastKnownLatitude = Results(i).Latitude
        LastKnownLongitude = Results(i).Longitude
    EndIf
Next

The above could should result in the following results, with the arrows pointing to the previously zero locations:

('2012/03/06 11:00', 1, 2), <-- 
('2012/03/06 11:01', 1, 2), <--
('2012/03/06 11:02', 2, 2),
('2012/03/06 11:03', 2, 2),
('2012/03/06 11:04', 2, 3),
('2012/03/06 11:20', 2, 3), <--
('2012/03/06 11:21', 5, 5),
('2012/03/06 11:22', 5, 6),
('2012/03/06 11:23', 5, 6),
('2012/03/06 11:24', 5, 7),
('2012/03/06 11:25', 5, 8)

How would I go about implementing this same functionality in SQL?

Addendum: Thanks for all the responses so far. I ought to point out that I am not interested in updating the original table – I would only like to modify the results of the query. The reason for this is because the rows in the table are not added in chronological order.

Answer 1

You can subquery table to get last knowed position:

select 
 [timestamp],
 case 
   when latitude>0 then latitude
   else (select top 1 latitude 
         from @VehicleData v2
         where 
           latitude > 0 and
           v2.[TimeStamp] < v1.[TimeStamp]
         order by v2.[TimeStamp] desc )
   end
   as latitude
from 
 @VehicleData v1

The sample is for latitude, do the same subquery to get longitude.