[SOLVED] GROUP BY with MAX(DATE)

GROUP BY with MAX(DATE)

I'm trying to list the latest destination (MAX departure time) for each train in a table, for example:

Train    Dest      Time
1        HK        10:00
1        SH        12:00
1        SZ        14:00
2        HK        13:00
2        SH        09:00
2        SZ        07:00

The desired result should be:

Train    Dest      Time
1        SZ        14:00
2        HK        13:00

I have tried using

SELECT Train, Dest, MAX(Time)
FROM TrainTable
GROUP BY Train

by I got a "ora-00979 not a GROUP BY expression" error saying that I must include 'Dest' in my group by statement. But surely that's not what I want...

Is it possible to do it in one line of SQL?

Solution

You cannot include non-aggregated columns in your result set which are not grouped. If a train has only one destination, then just add the destination column to your group by clause, otherwise you need to rethink your query.

Try:

SELECT t.Train, t.Dest, r.MaxTime
FROM (
      SELECT Train, MAX(Time) as MaxTime
      FROM TrainTable
      GROUP BY Train
) r
INNER JOIN TrainTable t
ON t.Train = r.Train AND t.Time = r.MaxTime