Jakarta Persistence Query Language

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Jakarta Persistence Query Language
OS	Cross-platform
Website	eclipse-ee4j.github.io/jakartaee-tutorial/#the-jakarta-persistence-query-language
Influenced by
SQL, Hibernate

The Jakarta Persistence Query Language (JPQL; formerly Java Persistence Query Language) is a platform-independent object-oriented query language ^[1]^{: 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,^[1]^{: 17, §1.3} but operate against JPA entity objects rather than directly with database tables.^[1]^{: 26, §2.2.3}

In addition to retrieving objects (SELECT queries), JPQL supports set based UPDATE and DELETE queries.

Examples

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Example JPA Classes, getters and setters omitted for simplicity.

@Entity
publicclass Author{
@Id
privateIntegerid;
privateStringfirstName;
privateStringlastName;

@ManyToMany
privateList<Book>books;
}

@Entity
publicclass Book{
@Id
privateIntegerid;
privateStringtitle;
privateStringisbn;

@ManyToOne
privatePublisherpublisher;

@ManyToMany
privateList<Author>authors;
}

@Entity
publicclass Publisher{
@Id
privateIntegerid;
privateStringname;
privateStringaddress;

@OneToMany(mappedBy="publisher")
privateList<Book>books;
}

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

SELECTaFROMAuthoraORDERBYa.firstName,a.lastName

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

SELECTDISTINCTaFROMAuthoraINNERJOINa.booksbWHEREb.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:

importjavax.persistence.EntityManager;
importjavax.persistence.TypedQuery;
...
publicList<Author>getAuthorsByLastName(StringlastName){
StringqueryString="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);
returnquery.getResultList();
}

Hibernate Query Language

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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.