Question

I have a table like this:

OrderID | PhaseID | Timestamp
1       | 1       | 1/1
1       | 2       | 1/2
1       | 3       | 1/3
1       | 2       | 1/4
1       | 4       | 1/5

I’m trying to get a query to return the most recent timestamp for each orderphase combination without being followed by a lesser phaseid. Something like this:

OrderID | PhaseID | MaxTimestampWithoutBeingFollowedByLesserPhaseID
1       | 1       | 1/1
1       | 2       | 1/4
1       | 3       | NULL
1       | 4       | 1/5

I keep running around in circles and coming up with this problem of a conditional aggregate query.

Can anybody figure out the query or give me some pointers?

Answer 1

With a as (
Select OrderID, PhaseID, MaxTimestamp=max([Timestamp])
From orderphase op
Where not exists(select 1 from orderphase where PhaseID < op.PhaseID 
    and [Timstamp]> op.[Timestamp])
Group by OrderID, PhaseID
)
Select distinct o.Orderid, o.PhaseID, MaxTimestamp=a.MaxTimestamp
From orderphase o
Left join a on a.OrderID = o.OrderID and a.PhaseID=o.PhaseID

EDIT ref a.MaxTimestamp