Question

OK, I have mapped with annotations two tables with a bidirectional @ManyToMany relationship.

Now I want to return only the elements which aren’t in a many to many relationship, and I’m trying to use the code from here, but It throws an exception at runtime.
Here’s the HQL:

String hql = "select a from Article a " +
                "left join a.tags t " +
                "group by a " +
                "having count(t)=0";

Is there a better way to return those elements? Or to fix the error in this query?

The exception it throws now is:

column "article0_.id" must appear in the GROUP BY clause or be used in an aggregate function

org.springframework.dao.InvalidDataAccessResourceUsageException: could not execute query; SQL ...

Answer 1

The query is correct, but on some databases group by a is not enough, you have to enumarate all properties of a, such as group by a.id, a.title.

Alternatively, you can use the following query:

select a from Article a where a.tags is empty

See also:

• 16.12. The group by clause