Spring Data JPA Select Distinct

Question

I have a situation where I need to build a select distinct a.address from Person a (where address is a Address entity inside the Person) kind of query.

I am using Specification to dynamically build my where clause and using the findAll(Specification<T>) function to get the result. The problem is I can not use specification to build my select clause and hence can not use findAll(Spcification) function.

What would be the best way to do something like this?

Answer 1

I came across the same issue, so in case it would help someone, this is what I did:

The Specification is being translated to the where clause, and the findAll(Specification<T>) function is creating its own select clause. So there is no way we can fix this by somehow using the findAll(Specification<T>) function. I already had custom repository which extends SimpleJpaRepository, so I've added a new method:

@Override
    @Transactional(readOnly = true)
    public List<Object> findDistinctValues(Specifications<T> spec, String columnName) {
        return getQuery(spec, columnName).getResultList();
    }

    protected TypedQuery<Object> getQuery(Specification<T> spec, final String distinctColumnName) {

        CriteriaBuilder builder = em.getCriteriaBuilder();
        CriteriaQuery<Object> query = builder.createQuery(Object.class);
        Root<T> root = applySpecificationToCriteria(spec, query);

        if (null != distinctColumnName) {
            query.distinct(true);
            query.multiselect(root.get(distinctColumnName));
        }

        // We order by the distinct column, Asc
        query.orderBy(builder.asc(root.get(distinctColumnName)));

        return em.createQuery(query);
    }

applySpecificationToCriteria is in the SimpleJpaRepository class.

Now you can use the findDistinctValues method.

Answer 2

As it's the top question in google, I'll post the answer here.

In the specification you have access to query, so you can do

query.distinct(true);

Full example, which results in such SQL emitted:

2015-04-27 12:03:39 EEST [7766-759] postgres@sales LOG: execute : SELECT DISTINCT t1.ID, t1.NAME, t1.WEBNAME, t1.WEBORDER, t1.PVGROUPPARENT_ID, t1.SITE_ID FROM PRODUCTVARIANT t0, PVGROUP t1 WHERE ((t0.PRODUCTTYPE_ID = $1) AND (t0.PVGROUP_ID = t1.ID)) 2015-04-27 12:03:39 EEST [7766-760] postgres@sales DETAIL: parameters: $1 = '4608bdc9-d0f2-4230-82fd-b0f776dc2cfd'

public static Specification<PVGroup> byProductType(final ProductType pt) {
        return (final Root<PVGroup> root, final CriteriaQuery<?> query, final CriteriaBuilder builder) -> {

            query.distinct(true);
            final CollectionJoin<PVGroup, ProductVariant> jPV = root.join(PVGroup_.productVariant);

            final Path<ProductType> ptPath = jPV.get(ProductVariant_.productType);

            return builder.equal(ptPath, pt);
        };
    }
}