Jakarta Persistence Query Language

Example JPA Classes, getters and setters omitted for simplicity.

@Entity public class Author { @Id private Integer id; private String firstName; private String lastName; @ManyToMany private List<Book> books; } @Entity public class Book { @Id private Integer id; private String title; private String isbn; @ManyToOne private Publisher publisher; @ManyToMany private List<Author> authors; } @Entity public class Publisher { @Id private Integer id; private String name; private String address; @OneToMany(mappedBy = "publisher") private List<Book> books; }

Then a simple query to retrieve the list of all authors, ordered alphabetically, would be:

SELECT a FROM Author a ORDER BY a.firstName, a.lastName

To retrieve the list of authors that have ever been published by XYZ Press:

SELECT DISTINCT a FROM Author a INNER JOIN a.books b WHERE b.publisher.name = 'XYZ Press'

JPQL supports named parameters, which begin with the colon (:). We could write a function returning a list of authors with the given last name as follows:

import javax.persistence.EntityManager; import javax.persistence.TypedQuery; ... public List<Author> getAuthorsByLastName(String lastName) { String queryString = "SELECT a FROM Author a " + "WHERE a.lastName IS NULL OR LOWER(a.lastName) = LOWER(:lastName)"; TypedQuery<Author> query = getEntityManager().createQuery(queryString, Author.class); query.setParameter("lastName", lastName); return query.getResultList(); }

JPQL is based on the Hibernate Query Language (HQL), an earlier non-standard query language included in the Hibernate object-relational mapping library.

Hibernate and the HQL were created before the JPA specification. As of Hibernate 3 JPQL is a subset of HQL.