Is there a "DISTINCT ON" equivalent in SPARQL? - sparql

Question

My data is basically an event log in RDF. I have cases and events, the latter belong to the former. Events have timestamps and an actor who triggered them.
For each case I now need the latest event, when it happened, and who triggered it.
This is roughly my current query:
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX ex: <http://example.org/>
SELECT ?case ?event ?timestamp ?actor
WHERE {
?case rdf:type ex:Case ;
ex:hasEvent ?event .
?event ex:timestamp ?timestamp ;
ex:hasActor ?actor .
}
ORDER BY ASC(?case) DESC(?timestamp)
Which yields something like this:
| case | event | timestamp | actor |
=================================================================================
| ex:case1 | ex:event1 | "2020-01-01T02:00:00Z"^^xsd:dateTimeStamp | ex:Alice |
| ex:case1 | ex:event2 | "2020-01-01T01:00:00Z"^^xsd:dateTimeStamp | ex:Bob |
| ex:case2 | ex:event3 | "2020-01-01T03:00:00Z"^^xsd:dateTimeStamp | ex:Charlie |
| ex:case2 | ex:event4 | "2020-01-01T02:00:00Z"^^xsd:dateTimeStamp | ex:Dan |
However I would like to only get the first and third row, as they correspond to the latest events for this case. Like this:
| case | event | timestamp | actor |
=================================================================================
| ex:case1 | ex:event1 | "2020-01-01T02:00:00Z"^^xsd:dateTimeStamp | ex:Alice |
| ex:case2 | ex:event3 | "2020-01-01T03:00:00Z"^^xsd:dateTimeStamp | ex:Charlie |
In order to achieve this I tried to use SELECT ?case ?event (MAX(?timestamp) AS ?latest) ?actor combined with GROUP BY ?case however SPARQL complains I need to group by ?event and ?actor as well which is not what I want of course.
I am aware that PostgreSQL has DISTINCT ON which would solve my problem, but I need to do it in SPARQL. Is there a nice way to achieve this?

Answer 1

Self answer based on #UninformedUser's comment:
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX ex: <http://example.org/>
SELECT ?case ?event (?latest as ?timestamp) ?actor WHERE {
?case ex:hasEvent ?event .
?event ex:timestamp ?latest ;
ex:hasActor?actor .
{ SELECT ?case (MAX(?timestamp) AS ?latest) {
?case rdf:type ex:case ;
ex:hasEvent ?event .
?event ex:timestamp ?timestamp }
group by ?case }
}