QueryDSL and SubQuery with Tuple condition

Question

I am trying to write a query in QueryDSL to fetch the oldest elements of a table grouped by their parentId.

The SQL equivalent should be:

SELECT a.* FROM child a
    INNER JOIN
    (
        SELECT parentId, MAX(revision) FROM child GROUP BY parentId
    ) b
    ON ( 
        a.parentId = b.parentId AND a.revision = b.revision
    )

Now in QueryDSL I'm stuck with the syntax.

JPQLQuery<Tuple> subquery = JPAExpressions
                .select(child.parent, child.revision.max())
                .from(child)
                .groupBy(child.parent);

HibernateQuery<Child> query = new HibernateQuery<>(session);
query.from(child)
    .where(child.parent.eq(subquery.???).and(child.revision.eq(subquery.???))));

How do you write this query using a subquery ?

The tables are looking like this :

___parent___ (not used in this query, but exists)
parentId
P1       | *
P2       | *
P3       | *

___child___
parentId | revision
P1       | 1       | *
P1       | 2       | *
P1       | 3       | *
P2       | 2       | *
P2       | 3       | *
P3       | 1       | *

___result from child, the highest revision for each parentId___
P1       | 3       | *
P2       | 3       | *
P3       | 1       | *

What I've tried so far :

.where(JPAExpressions.select(child.parent,child.revision).eq(subquery));

-> org.hibernate.hql.internal.ast.QuerySyntaxException: unexpected end of subtree

and many syntax errors ...

I use a dirty loop, for now, since I haven't found a solution yet.

Answer 1

You can use Expressions.list() to specify more than one column for the in clause:

query.from(child).where(Expressions.list(child.parent, child.revision).in(subquery));

The alternative is to use innerJoin(), as in your original SQL.