How to count distinct items on specific fields in QueryDSL
Solution 1
This is not a QueryDSL limitation, but a JPA limitation. The solution was to rewrite in SQL.
Solution 2
You can do query.select(q.field.countDistinct())
Adrian Cox
Consultant specialising in connecting embedded systems to the web and the desktop.
Updated on July 20, 2022Comments
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Adrian Cox almost 2 years
Edit: it turns out that JPA can't express this. The solution was to rewrite in SQL.
I'm using QueryDSL to perform an aggregate query on a JPA data set for reporting. I have no problem extracting the report data. For example:
... query = query.groupBy(QVehicle.vehicle.make, QVehicle.vehicle.model); return query.listDistinct(new QMakeModelReportData( QVehicle.vehicle.make, QVehicle.vehicle.model, QVehicle.vehicle.make.count()));
This produces a list of my DTO object, each of which contains a vehicle make, vehicle model, and the count of vehicles of that make model. Like this:
Ford, Focus, 14 Ford, Mondeo, 4 Vauxhall, Astra, 4
But I can't work out the syntax to count the number of rows before I actually perform the query. The syntax I imagine is like this, which doesn't exist:
return query.countDistinct(QVehicle.vehicle.make, QVehicle.vehicle.model);
I've ended up with a rather inefficient option:
return query .listDistinct(QVehicle.vehicle.make, QVehicle.vehicle.model) .size();
Is there anything better?
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Adrian Cox over 7 yearsI am attempting to count distinct on multiple fields, not a single field. JPA is limited to performing count distinct on a single field only: stackoverflow.com/a/37784831/184998