按数量查询的变压器组

Question

I am stuck trying to get a query (QueryDSL) to work that gives me a count of distinct categories. For example, what I am trying to achieve:

categoryA -> 10 entries
categoryB -> 20 entries

This is what i have so far:

query().from(application)
            .transform(groupBy(application.category).as(list(application)));

However, this gives me for each category a list of all whole entries, I just want to get a count of this.

I tried messing around with count() but no luck.

Anybody know how to do this?

Answer 1

Note that as of Querydsl 4.x , the accepted answer by Timo Westkämper is no longer valid. Breaking changes in Querydsl 4.0 have removed the query.list() method .

A solution working with Querydsl 4.0+ would now be:

query.from(application)
     .groupBy(application.category)
     .select(application.category, application.category.count())
     .fetch();

Answer 2

transform combined groupBy is meant to construct tree structures out of flat result sets. What you need can be easier expressed as

query.from(application)
     .groupBy(application.category)
     .list(application.category, application.category.count())

Answer 3

If using 4.1.4, you may need to use a JPAQueryFactory instead of JPAQuery

If using JPAQuery, there is no select() or fetch() on the return from groupBy.

JPAQueryFactory queryFactory = new JPAQueryFactory(entityManager);

queryFactory
     .select(application.category, application.category.count())
     .from(application)
     .groupBy(application.category)
     .fetch();