DataNucleus Access Platform - JPA Queries

The JPA specification defines JPQL (a pseudo-OO query language, with SQL-like syntax), for selecting objects from the datastore. To provide a simple example, this is what you would do

Query q = em.createQuery("SELECT p FROM Person p WHERE p.lastName = 'Jones'");
List results = (List)q.getResultsList();

This finds all "Person" objects with surname of "Jones". You specify all details in the query.

SELECT Syntax

In JPQL queries you define the query in a single string, defining the result, the candidate class(es), the filter, any grouping, and the ordering. This string has to follow the following pattern

SELECT [<result>]
    [FROM <candidate-class(es)>]
    [WHERE <filter>]
    [GROUP BY <grouping>]
    [HAVING <having>]
    [ORDER BY <ordering>]

The "keywords" in the query are shown in UPPER CASE are case-insensitive.

Entity Name

In the example shown you note that we did not specify the full class name. We used Person p and thereafter could refer to p as the alias. The Person is called the entity name and in JPA MetaData this can be defined against each class in its definition. For example

<entity class="org.datanucleus.company.Person" name="Person">
    ...
</entity>

Input Parameters

In JPQL queries it is convenient to pass in parameters so we dont have to define the same query for different values. Let's take two examples

Named Parameters :
Query q = em.createQuery("SELECT p FROM Person p WHERE p.lastName = :surname AND o.firstName = :forename");
q.setParameter("surname", theSurname);
q.setParameter("forename", theForename");

Numbered Parameters :
Query q = em.createQuery("SELECT p FROM Person p WHERE p.lastName = ?1 AND p.firstName = ?2");
q.setParameter(1, theSurname);
q.setParameter(2, theForename);

So in the first case we have parameters that are prefixed by : (colon) to identify them as a parameter and we use that name when calling Query.setParameter() . In the second case we have parameters that are prefixed by ? (question mark) and are numbered starting at 1. We then use the numbered position when calling Query.setParameter() .

Range of Results

With JPQL you can select the range of results to be returned. For example if you have a web page and you are paginating the results of some search, you may want to get the results from a query in blocks of 20 say, with results 0 to 19 on the first page, then 20 to 39, etc. You can facilitate this as follows

Query q = em.createQuery("SELECT p FROM Person p WHERE p.age &gt; 20");
q.setFirstResult(0);
q.setMaxResults(20);

So with this query we get results 0 to 19 inclusive.

Query Execution

There are two ways to execute a JPQL query. When you know it will return 0 or 1 results you call

Object result = query.getSingleResult();

If however you know that the query will return multiple results, or you just don't know then you would call

List results = query.getResultList();