Subselect, Group by and Having Count with Criteria in Hibernate

Introduction

Hibernate is a popular object-relational mapping (ORM) tool for Java applications. One of the common use cases in Hibernate is to perform complex queries involving subselections, grouping, and aggregations. In this blog post, we will explore how to write a query using subselect, group by, and having count with criteria in Hibernate.

Understanding Subselect in Hibernate

Subselect, also known as correlated subquery or nested query, allows us to use a subquery within the outer query. This can be useful when we need to filter or conditionally select rows based on some conditions. In our example, we want to write a query that selects distinct person_id and user_id from the zrm_actor table where deleted != 1, but also group by person_id and have a count of more than 1.

Limitations of Subselect in Hibernate

Unfortunately, subselect with criteria is not directly supported in Hibernate. The reason for this limitation is that subqueries can lead to complex queries that may not be efficient or scalable. In our example, the query would involve correlated subqueries which are notoriously hard to optimize.

## Query Structure and Limitations

The provided query:
```sql
SELECT ap.person_id FROM 
    (SELECT distinct ac.person_id person_id,ac.user_id 
     FROM zrm_actor ac WHERE ac.deleted != 1) ap 
WHERE person_id IS NOT NULL 
GROUP BY person_id HAVING COUNT(*) > 1;

is not directly supported in Hibernate. This is because subselect with criteria can lead to complex queries that may not be efficiently executed.

Using Subselect, Group by and Having Count without Criteria

To write this query without using subselect with criteria, we need to use the @Criteria annotation on our entity class or a CriteriaQuery object in our Java code. This allows us to specify the conditions for the group by clause.

## Example Entity Class with @Criteria Annotation

```java
@Entity
@Table(name = "zrm_actor")
public class ZrmActor {

    @Id
    private Long id;

    private String personId;
    private String userId;

    // Getters and Setters

    @Transient
    public List<ZrmActor> getDistinctPersonIds() {
        return em.createQuery("SELECT DISTINCT zra.personId FROM ZrmActor zra WHERE zra.deleted = false")
                .getResultList();
    }
}

Using Distinct with CriteriaQuery

We can use the distinct keyword along with our CriteriaBuilder object to create a query that groups by distinct columns.

## CriteriaQuery for Subselect, Group by and Having Count

```java
public List<ZrmActor> getDistinctPersonIds() {
    final CriteriaBuilder cb = em.getCriteriaBuilder();
    final CriteriaQuery<ZrmActor> cq = em.createQuery("SELECT DISTINCT zra FROM ZrmActor zra WHERE zra.deleted = false", ZrmActor.class);

    cq.where(cb.and(
            cb.like(cb.literal("%"), "personId"),
            cb.isNot(null(), "userId")
    ));

    return cq.getResultList();
}

Using Subselect with Criteria

If we still need to use a subquery, we can do so by using the having keyword along with our CriteriaBuilder object.

## Subselect with CriteriaQuery

```java
public List<ZrmActor> getSubSelectGroupByAndHavingCount() {
    final CriteriaBuilder cb = em.getCriteriaBuilder();
    final CriteriaQuery<ZrmActor> cq = em.createQuery("SELECT zra FROM ZrmActor zra", ZrmActor.class);

    cq.where(cb.and(
            cb.isNotNull(cb.col("personId")),
            cb.greaterThan(cb.col("count"), 1)
    ));

    return cq.getResultList();
}

Note that this approach may not be the most efficient for large datasets.

Conclusion

In this blog post, we explored how to write a query using subselect, group by, and having count with criteria in Hibernate. We discovered that subselect with criteria is not directly supported in Hibernate due to performance limitations. Instead, we used various techniques such as @Criteria annotation on our entity class or CriteriaQuery objects with distinct, having keywords along with our CriteriaBuilder object.

While these methods can be used to achieve the desired results, they come with their own trade-offs and may not be suitable for all use cases. When working with complex queries in Hibernate, it’s essential to understand the limitations and best practices of each approach.

Last modified on 2023-06-10