From Wikipedia, the free encyclopedia
The Jakarta Persistence Query Language (JPQL; formerly Java Persistence Query Language) is a platform-independent object-oriented query language: 284, §12 defined as part of the Jakarta Persistence (JPA; formerly Java Persistence API) specification.
JPQL is used to make queries against entities stored in a relational database. It is heavily inspired by SQL, and its queries resemble SQL queries in syntax,: 17, §1.3 but operate against JPA entity objects rather than directly with database tables.: 26, §2.2.3
In addition to retrieving objects (SELECT queries), JPQL supports set based UPDATE and DELETE queries.
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();
}
Hibernate Query Language[edit]
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.
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