Return only one row when multiple rows exist - sql

Question

I am working with a database that tracks field trip information for schools. The query will run on a MS SQL 2005 Server. There are a few cases when my query will return multiple rows for the same Field Trip. So, what I want to do is filter my results so that if more than one row per TripID is returned, display only the row with the MIN StartDateTime.
I know there's something I can do with the PARTITION and MIN functions but I'm not sure how to go about it.
Here is my code:
SELECT DISTINCT
dbo.Trip_TripInformation.RecordID AS TripID,
dbo.Trip_TripInformation.TripDate,
Origin.LocationName AS Origin,
dbo.Trip_TripInformation.OriginDepartureTime AS StartDateTime,
dbo.Trip_TripInformation.OriginReturnTime AS ReturnDateTime,
ISNULL(dbo.Trip_TripInformation.NoOfStudents, 0) AS NumberOfStudents,
ISNULL(dbo.Trip_TripInformation.NoOfAdults, 0) AS NumberOfAdults,
ISNULL(dbo.Trip_TripInformation.NoOfStudents, 0) + ISNULL(dbo.Trip_TripInformation.NoOfAdults, 0) AS NumberOfPassengers,
Destination.LocationName AS Destination,
dbo.Vehicles.Vehicle,
Driver.LastName + ', ' + Driver.FirstName AS Driver
FROM dbo.Trip_TripInformation
LEFT JOIN dbo.Trip_Location AS Origin ON Origin.RecordID = dbo.Trip_TripInformation.OriginLocationID
LEFT JOIN dbo.Trip_TripDestinations ON dbo.Trip_TripInformation.RecordID = dbo.Trip_TripDestinations.TripID
LEFT JOIN dbo.Trip_Location AS Destination ON Destination.RecordID = dbo.Trip_TripDestinations.LocationID
LEFT JOIN dbo.Trip_TripDriverVehicle ON dbo.Trip_TripInformation.RecordID = dbo.Trip_TripDriverVehicle.TripID
AND dbo.Trip_TripDriverVehicle.DestinationID = dbo.Trip_TripDestinations.RecordID
LEFT JOIN dbo.Vehicles ON dbo.Vehicles.RecordID = dbo.Trip_TripDriverVehicle.VehicleID
LEFT JOIN dbo.Employees AS Driver ON dbo.Trip_TripDriverVehicle.DriverID = Driver.RecordID
ORDER BY TripID

Answer 1

Order by StartDate and then select the TOP(1)

Answer 2

Try adding a row number to your select and selecting your data into a temp table (or use a CTE):
ROW_NUMBER() OVER ( PARTITION BY dbo.Trip_TripInformation.RecordID
ORDER BY dbo.Trip_TripInformation.OriginDepartureTime asc
) as RowNum
Then you can just select from that where RowNum = 1

Answer 3

You need to GROUP them.
Happy coding

Answer 4

You need to join to a derived table which extracts the unique TripId and earliest departure time for each trip:
SELECT DISTINCT
...
FROM dbo.Trip_TripInformation
INNER JOIN (
SELECT TripID, MIN(OriginDepartureTime) as EarliestDepartureTime
FROM Trip_TripInformation
GROUP BY TripID
) EarliestTripOnly
ON
Trip_TripInformation.TripID = EarliestTripOnly.TripId
AND
Trip_TripInformation.OriginDepartureTime
= EarliestTripOnly.EarliestDepartureTime
LEFT JOIN dbo.Trip_Location AS Origin ON Origin.RecordID =
dbo.Trip_TripInformation.OriginLocationID
...

Answer 5

You can use the Row_Number function to number each start date within each TripID. In addition, I encapsulated the query into a common-table expression so that I could then fitler on only those Trips where their row numbering was 1 which will represent the earliest date should a trip return multiple rows.
With TripInfo As
(
Select TInfo.RecordID As TripID
, TInfo.TripDate
, Origin.LocationName As Origin
, TInfo.OriginDepartureTime As StartDateTime
, TInfo.OriginReturnTime As ReturnDateTime
, Coalesce( TInfo.NoOfStudents, 0 ) As NumberOfStudents
, Coalesce( TInfo.NoOfAdults, 0) As NumberOfAdults
, Coalesce( TInfo.NoOfStudents, 0) + Coalesce(TInfo.NoOfAdults, 0) As NumberOfPassengers
, Dest.LocationName As Destination
, V.Vehicle
, Driver.LastName + ', ' + Driver.FirstName As Driver
, Row_Number() Over ( Partition By TInfo.RecordId
Order By TInfo.OriginDepartureTime ) As TripDateRnk
From dbo.Trip_TripInformation As TInfo
Left Join dbo.Trip_Location AS Origin
On Origin.RecordID = TInfo.OriginLocationID
Left Join dbo.Trip_TripDestinations As TDest
On TInfo.RecordID = TDest.TripID
Left Join dbo.Trip_Location AS Destination
On Destination.RecordID = TDest.LocationID
Left Join dbo.Trip_TripDriverVehicle As TripV
On TInfo.RecordID = TripV.TripID
And TripV.DestinationID = TDest.RecordID
Left Join dbo.Vehicles As V
ON dbo.Vehicles.RecordID = TripV.VehicleID
Left Join dbo.Employees AS Driver
On dbo.Trip_TripDriverVehicle.DriverID = Driver.RecordID
)
Select TripID, TripDate, Origin, StartDateTime, ReturnDateTime
, NumberOfStudents, NumberOfAdults, NumberOfPassengers
, Destination, Vehicle, Driver
From TripInfo
Where TripDateRank = 1
Order By TripID
A couple of other observations:
I notice that every table is using a Left Join. Is it really the case that all the columns in question are nullable? For example, is it really the case that the VehicleID and the DriverID columns in the Trip_TripDriverVehicle table are nullable? You can designate a trip driver vehicle without the vehicle and without a driver?
I would recommend using Coalesce function instead of the awfully named SQL Server-specific function IsNull. They operate pretty much the same but Coalesce is standard and allows for more than two parameters whereas IsNull is restricted to two. It won't make any significant difference with respect to your code or performance. It's just a style improvement.

Answer 6

SELECT * FROM
(SELECT DISTINCT RowNum = ROW_NUMBER() OVER (PARTITION BY TI.RecordID ORDER BY Trip_TripDestinations.DestinationArrivalTime),
TI.RecordID AS TripID,
TI.TripDate,
Origin.LocationName AS Origin,
TI.OriginDepartureTime AS StartDateTime,
TI.OriginReturnTime AS ReturnDateTime,
ISNULL(TI.NoOfStudents, 0) AS NumberOfStudents,
ISNULL(TI.NoOfAdults, 0) AS NumberOfAdults,
ISNULL(TI.NoOfStudents, 0) + ISNULL(TI.NoOfAdults, 0) AS NumberOfPassengers,
Destination.LocationName AS Destination,
Trip_TripDestinations.DestinationArrivalTime AS DestinationArrivalDateTime,
Vehicles.Vehicle,
Driver.LastName + ', ' + Driver.FirstName AS Driver
FROM Trip_TripInformation TI
LEFT OUTER JOIN Trip_Location AS Origin ON Origin.RecordID = TI.OriginLocationID
/*More Joins... */
LEFT OUTER JOIN Employees AS Driver ON Trip_TripDriverVehicle.DriverID = Driver.RecordID) Q1
WHERE Q1.RowNum = 1 and (Q1.TripDate BETWEEN '2010/12/13 00:00:00' AND '2010/12/17 00:00:00')
ORDER BY Q1.DestinationArrivalDateTime