Help with SQL query - some magic needed

Question

I need some help with a query - I'm using Firebird 2.1.

I have a table like:

RowID (primary key) | ActivityID | Duration | BilledAt

1 | 1 | 50 | 06.08.2010, 14:05:00.598
2 | 1 | 70 | 06.08.2010, 14:05:00.608
3 | 2 | 30 | 06.08.2010, 14:05:00.598
4 | 3 | 40 | 06.08.2010, 14:05:00.598
5 | 3 | 50 | 06.08.2010, 14:05:00.608

I'd like to get the Durations for each ActivityID BUT if there are more than one entries available with the same ActivityID, I need the get the one with the highest BilledAt value. (the most recent entry)

If I execute:

SELECT ActivityID, Max(BilledAt) 
FROM BilledTime 
GROUP BY ActivityID;

I'll get what I want without the Duration values. If I include the Duration column in the GROUP BY clause, then multiple ActivityIDs are selected.

Is there an elegant solution to this?

Thanks!

Answer 1

Not familiar with Firebird so the syntax might be wrong, but this should work:

SELECT a.ActivityID, a.Duration, a.BilledAt
FROM BilledTime a
LEFT JOIN BilledTime b on a.ActivityID = b.ActivityID AND b.BilledAt > a.BilledAt
WHERE b.ActivityID IS NULL

Alternatively you can use a more intuitive WHERE NOT EXISTS subquery instead of the LEFT JOIN, but I believe the above ends up being faster.

Answer 2

I would do like this

SELECT a.ActivityID, a.Duration, a.BilledAt 
FROM BilledTime a 
WHERE a.BilledAt = (select max(b.billedAt) from BilledTime b where b.ActivityId = a.ActivityID)